Animal Viruses

Bacteriology at UW- Madison

The Microbial World

University of Wisconsin - Madison

Animal Viruses

Influenza Virus H5N1 (avian influenza)

General Features of Viruses

Viruses consist of nucleic acid (DNA or RNA) surrounded by a protein coat called a capsid. The capsid is made up of individual structural subunits called capsomeres. The combination of the nucleic acid genome enclosed in the capsid is called the nucleocapsid. In addition, many animal viruses have an envelope, which is a membranous lipid structure that surrounds the nucleocapsid.

The structural components of a Herpes virus are illustrated below.

Herpes simplex Virus 1, illustrating the basic structural features of a virus. HSV1 is an enveloped, icosahedral DNA virus. The region between the outer lipid envelope and the nucleocapsid is called the tegument. The DNA of the virus resides in the core. The envelope proteins ("Glycoprotein Spikes") are unique viral proteins, but the envelope itself is derived from the virus host cell.

Viruses are quite different from cells. They contain only one type of nucleic acid, DNA or RNA, never both. They lack membranes and a cytoplasm, as well as ribosomes and any means to produce energy. Although viruses can replicate, mutate and maintain genetic continuity, which are features of all cells, they depend entirely upon a host cell to supply a habitat, energy and raw materials (precursors) for viral replication. Thus, viruses must exist as obligate intracellular parasites of cells.

Viruses are very small in size. Some are not as large as a cell ribosome. Their size is so small that individual virus particles cannot be visualized with the light microscope. The range of particle size is from about 20 nanometers for a small virus (e.g. poliovirus) to about 0.3 micrometers for a very large virus [e.g. smallpox (variola) virus].

Animal viruses have many shapes ranging from cubical, bullet-shaped, polygonal, spherical, filamentous or helical, to a complex layered morphology. One of the most common morphologies of the viral capsid is the icosahedron, which consists of 20 triangular faces (capsomeres) that coalesce to form a roughly spherical structure enclosing the viral nucleic acid. The herpes virus illustrated above has the icosahedral shape.

Common morphologies seen in animal viruses. Left to Right. A naked icosahedral virus (e.g. poliovirus), an enveloped icosahedral virus (e.g. herpes virus), a naked helical virus, and an enveloped helical virus (e.g. influenza virus). Individual capsomeres are arranged to form a capsid which encloses the nucleic acid (DNA or RNA) of the virus.

Classification of Viruses

The primary criteria for taxonomic classification of animal viruses are based on morphology (size, shape, etc.), type of nucleic acid (DNA, RNA, single-stranded, double-stranded, linear, circular, segmented, etc.), and occurrence of envelopes. ssRNA viruses possess either (+)RNA (if it serves as messenger RNA) or (-)RNA (if it serves as a template for messenger RNA). Host range is not a particularly reliable criterion for classification. Although some animal viruses exhibit a very narrow or specific host range, such as HIV in humans or canine distemper virus (CDV) in dogs. But for classification purposes, host range cannot be a criterion because each animal species is subject to infection by a wide variety of viral agents, and numerous viruses infect several different animal species. For example, West Nile virus has a primary host of birds, but it infects and causes disease in horses and humans. Some viruses, such as the influenza virus, are able to change their structure in such a way that they can shift from one primary host to another, for example birds to humans.

Morphologic similarity among animal viruses correlates closely with similarity of viral components, particularly with the type and size of the viral nucleic acid (genome). For example, all viruses with the morphology of adenoviruses contain dsDNA genomes with a molecular weight of about 23 million daltons; all reoviruses contain segmented dsRNA genomes. In fact, a system of virus classification based on structure and size of viral genomes yields that same grouping as one based on morphology. This information is organized in two ways.

According to the Baltimore method of classification, animal viruses are be separated into several classes, grouped by type of nucleic acid. Class I. dsDNA viruses; Class II. ssDNA viruses; Class III. dsRNA viruses; Class IV. (+)RNA viruses; Class V. (-)RNA viruses: Class VI. RNA reverse transcribing viruses; Class VII. DNA reverse transcribing viruses. The Baltimore method of classification is illustrated in the table below.

Baltimore Method of classification of animal viruses, grouped by genome structure. This method classifies viruses with regard to the various mechanisms of viral genome replication. The central theme is that all viruses must generate positive strand mRNAs [(+) RNA] from their genomes, in order to produce proteins and replicate themselves. The precise mechanisms whereby this is achieved differ for each virus family. These various types of virus genomes can be broken down into seven strategies for their replication. For a more complete listing of family groups of viruses classified by the Baltimore method, please see www.virology.net/BigVirology/BVFamilyGroup.html

On the basis of morphology alone, animal viruses are organized into a hierarchical scheme consisting of virus families and constitutive genera based on size, shape, type of nucleic acid and the presence or absence of an envelope. Some families of viruses generated in this scheme are described and illustrated below.

Some families of Animal Viruses

Replication of Animal Viruses

Outside its host cell a virus is an inert particle. However, when it encounters a host cell it becomes a highly efficient replicating machine. After attachment and gaining entry into its host cell, the virus subverts the biosynthetic and protein synthesizing abilities of the cell in order to replicate the viral nucleic acid, make viral proteins and arrange its escape from the cell. The process occurs in several stages and differs in its details among DNA-containing and RNA-containing viruses.

The Stages of Replication

1. The first stage in viral replication is called the attachment (adsorption) stage. Like bacteriophages, animal viruses attach to host cells by means of a complementary association between attachment sites on the surface of the virus and receptor sites on the host cell surface. This accounts for specificity of viruses for their host cells. Attachment sites on the viruses (usually called virus receptors) are distributed over the surface of the virus coat (capsid) or envelope, and are usually in the form of glycoproteins or proteins. Receptors on the host cell (called the host cell receptors) are generally glycoproteins imbedded into the cell membrane. Cells lacking receptors for a certain virus are resistant to it and cannot be infected. Attachment can be blocked by antibody molecules that bind to viral attachment sites or to host cell receptors. Since antibodies block the initial attachment of viruses to their host cells, the presence of these antibodies in the host organism are the most important basis for immunization against viral infections.

2. The penetration stage follows attachment. Penetration of the virus occurs either by engulfment of the whole virus, or by fusion of the viral envelope with the cell membrane allowing only the nucleocapsid of the virus to enter the cell. Animal viruses generally do not "inject" their nucleic acid into host cells as do bacteriophages, although occasionally non enveloped viruses leave their capsid outside the cell while the genome passes into the cell.

3. Once the nucleocapsid gains entry into the host cell cytoplasm, the process of uncoating occurs. The viral nucleic acid is released from its coat. Uncoating processes are apparently quite variable and only poorly understood. Most viruses enter the host cell in an engulfment process called receptor mediated endocytosis and actually penetrate the cell contained in a membranous structure called an endosome. Acidification of the endosome is known to cause rearrangements in the virus coat proteins which probably allows extrusion of the viral core into the cytoplasm. Some antiviral drugs such as amantadine exert their antiviral effect my preventing uncoating of the viral nucleic acid.

4. Immediately following uncoating, the viral synthesis stage begins. Exactly how these events will unfold depends upon whether the infecting nucleic acid is DNA or RNA.

In DNA viruses, such as Herpes, the viral DNA is released into the nucleus of the host cell where it is transcribed into early mRNA for transport into the cytoplasm where it is translated into early viral proteins. The early viral proteins are concerned with replication od the viral DNA, so they are transported back into the nucleus where they become involved in the synthesis of multiple copies of viral DNA. These copies of the viral genome are then templates for transcription into late mRNAs which are also transported back into the cytoplasm for translation into late viral proteins. The late proteins are structural proteins (e.g. coat, envelope proteins) or core proteins (certain enzymes) which are then transported back into the nucleus for the next stage of the replication cycle.

In the case of some RNA viruses (e.g. picornaviruses), the viral genome (RNA) stays in the cytoplasm where it mediates its own replication and translation into viral proteins. In other cases (e.g. orthomyxoviruses), the infectious viral RNA enters into the nucleus where it is replicated before transport back to the cytoplasm for translation into viral proteins.

5. Once the synthesis of the various viral components is complete, the assembly stage begins. The capsomere proteins enclose the nucleic acid to form the viral nucleocapsid. The process is called encapsidation. If the virus contains an envelope it will acquire that envelope and asssociated viral proteins in the next step.

6. The release stage is the final event in viral replication, and it results in the exit of the mature virions from their host cell. Virus maturation and release occurs over a considerable period of time. Some viruses are released from the cell without cell death, by egestion, whereas others are released when the cell dies and disintegrates. In the case of enveloped viruses, the nucleocapsid acquires its final envelope from the nuclear or cell membrane by a budding off process (envelopment) before egress (exit) out of the host cell. Whenever a virus acquires a membrane envelope, it always inserts specific viral proteins into the that envelope which become unique viral antigens and which will be used by the virus to gain entry into a new host cell.

Below are illustrated the modes of replication of two viruses that conform to this model. Herpes simplex virus (HSV) is an enveloped, double stranded DNA virus; Influenza virus is an enveloped, single stranded (-)RNA virus that contains a segmented genome.

The replication cycle of Herpes Simplex virus. 1. Specific proteins in the viral envelope attach to host cell receptors on the cell membrane. 2. Penetration is achieved when the viral envelope fuses with the cell membrane releasing the nucleocapsid directly into the cytoplasm. 3. The virion is uncoated and the viral DNA is transported into the nucleus. 4. In the nucleus, the viral DNA is transcribed into early mRNAs which are transported to the cytoplasm for the translation of early proteins. These early proteins are brought back into the nucleus and participate in the replication of the virus DNA into many copies. The viral DNA is then transcribed into the late mRNAs which exit to the cytoplasm for translation into the late (nucleocapsid and envelope) proteins. 5. The capsid proteins encapsidate the newly replicated genomes. The envelope proteins are imbedded in the nuclear membrane. 6. The nucleocapsids are enveloped by budding through the nuclear membrane, and the mature viruses are released from the cell through cytoplasmic channels. To view an animation of the life cycle of Herpes go to the Homepage of Dr. Edward K. Wagner at U Cal Irvine

The replication cycle of Influenza A Virus. Diagram from accessexcellence.org 1. The virus adsorbs to the cell surface by means of specific receptors. 2. The virus is taken up in a membrane enclosed endosome by the process of receptor mediated endocytosis. 3. Uncoating takes place in the endosome and the viral RNA (genome) is released into the cytoplasm. 4. The (-)RNA of the viral genome is transported into the nucleus where it is replicated and copied by a viral enzyme into (+)RNA which is both messenger RNA and serves as a template for more (-)RNA. The (+)RNA is transported into the cytoplasm for translation into early and late viral proteins. 5. The viral core proteins are transported back into the nucleus to assemble as the capsid around the viral (-)RNA forming the "ribonucleoprotein core" or the genome-containing nucleocapsid of the virus. The viral envelope proteins assemble themselves in the cell membrane. 6. The nucleocapsid recognizes specific points on cell membrane where viral proteins have become inserted and buds off of the membrane to be released during enclosure in the viral envelope.

How Viruses Cause Disease

Their are several possible consequences to a cell that is infected by a virus, and ultimately this may determine the pathology of a disease caused by the virus.

Lytic infections result in the destruction of the host cell. Lytic infections are caused by virulent viruses, which inherently bring about the death of the cells that they infect.

When enveloped viruses are formed by budding, the release of the viral particles may be slow and the host cell may not be lysed. Such infections may occur over relatively long periods of time and are thus referred to as persistent infections.

Viruses may also cause latent infections. The effect of a latent infection is that there is a delay between the infection by the virus and the appearance of symptoms. Fever blisters (cold sores) caused by herpes simplex type 1 result from a latent infection; they appear sporadically as the virus emerges from latency, usually triggered by some sort of stress in the host.

Some animal viruses have the potential to change a cell from a normal cell into a tumor cell, the hallmark of which is to grow without restraint. This process is called transformation. Viruses that are able to transform normal cells into tumor cells are referred to as oncogenic viruses and their role in causing cancer in humans will be discussed later.

The possible effects that animal viruses may have on the cells that they infect.

The vast majority of viral infections in humans are inapparent or asymptomatic. Viral pathogenesis is the abnormal situation and it is of no particular value to the virus, although it typically results in the multiplication of the viruses that can be transmitted to other individuals. For pathogenic viruses, there are a number of critical stages in replication which determine the nature of the disease they produce.

The Stages of Viral Infections

1. Entry into the Host
The first stage in any virus infection, irrespective of whether the virus is pathogenic or not. In the case of pathogenic infections, the site of entry can influence the disease symptoms produced. Infection can occur via several portals of entry.

Skin - Most viruses which infect via the skin require a breach in the physical integrity of this effective barrier, e.g. cuts or abrasions. Some viruses employ vectors, e.g. ticks, mosquitos, etc. to breach the skin.

Respiratory tract - The respiratory tract and all other mucosal surfaces possess sophisticated immune defense mechanisms, as well as non-specific inhibitory mechanisms (ciliated epithelium, mucus secretion, lower temperature, etc.) which viruses must overcome. Nonetheless, this is the most common point of entry for most viral pathogens.

Gastrointestinal tract - a fairly protected mucosal surface, but some viruses (e.g. enteroviruses, including polioviruses) enter at this site.

Genitourinary tract - less protected than the GI tract, but less frequently exposed to extraneous viruses.

Conjunctiva - an exposed site and relatively unprotected.

2. Primary Replication

Having gained entry to a potential host, the virus must initiate an infection by entering a susceptible cell. Some viruses remain localized after primary infection, but others replicate at a primary site before dissemination and spread to a secondary site. Examples are given in the table below.

Localized Infections:
Virus:	Primary Replication:
Rhinoviruses	Upper respiratory tract
Rotaviruses	Intestinal epithelium
Papillomaviruses	Epidermis
Systemic Infections:
Virus:	Primary Replication:	Secondary Replication:
Enteroviruses (poliovirus)	Intestinal epithelium	Lymphoid tissues, CNS
Herpesvirus (HSV types 1 and 2)	Oropharynx or urogenital tract	Lymphoid cells, peripheral nervous system, CNS
Rabies virus	Muscle cells and connective tissue	CNS

3. Dissemination Stage
There are two main mechanisms for viral spread throughout the host: via the bloodstream and via the nervous system.

The virus may get into the bloodstream by direct inoculation - e.g. arthropod vectors, blood transfusion or I.V. drug abuse. The virus may travel free in the plasma (Togaviruses, Enteroviruses), or in association with red cells (Orbiviruses), platelets (HSV), lymphocytes (EBV, CMV) or monocytes (Lentiviruses). the presence of viruses in the bloodstream is referred to as a viremia. Primary viremia may be followed by more generalized secondary viremia as the virus reaches other target tissues or replicates directly in blood cells.

In some cases, spread to nervous system is preceded by primary viremia, as above. In other cases, spread occurs directly by contact with neurons at the primary site of infection. Once in peripheral nerves, the virus can spread to the CNS by axonal transport along neurons (e.g. HSV). Viruses can cross synaptic junctions since these frequently contain virus receptors, allowing the virus to jump from one cell to another.

4. Tissue/Cell tropism
Tropism is the ability of a virus to replicate in particular cells or tissues. It is influenced partly by the route of infection but largely by the interaction of a virus attachment sites (virus receptors) with specific receptors on the surface of a cell. The interaction of the virus receptors with the host cell receptors may have a considerable effect on pathogenesis.

5. Host Immune Responses
There are several ways that the host immune responses may contribute to viral pathology. The mechanisms of cell mediated immunity are designed to kill cells which are infected with viruses. If the mechanisms of antibody mediated immunity result in the production of antibodies that cross-react with tissues, an autoimmune pathology may result.

6. Secondary Replication
This occurs in systemic infections when a virus reaches other tissues in which it is capable of replication. For example, polioviruses initiate infection in the GI where the produce an asymptomatic infection. However, when disseminated to neurons in the brain and spinal cord, where the virus replicates secondarily, the serious paralytic complication of poliomyelitis occurs. If a virus can be prevented from reaching tissues where secondary replication can occur, generally no disease results.

7. Direct Cell and Tissue Damage
Viruses may replicate widely throughout the body without any disease symptoms if they do not cause significant cell damage or death. Although retroviruses (e.g. HIV) do not generally cause cell death, being released from the cell by budding rather than by cell lysis, they cause persistent infections and may be passed vertically to offspring if they infect the germ line. Conversely, most other viruses, referred to as virulent viruses, ultimately damage or kill their host cell by several mechanisms, including inhibition of synthesis of host cell macromolecules, damage to cell lysosomes, alterations of the cell membrane, development of inclusion bodies, and induction of chromosomal aberrations.

8. Persistence versus Clearance
The eventual outcome of any virus infection depends on a balance between the ability of the virus to persist or remain latent (persistence) and the forces of the host to completely eliminate the virus (clearance).

Long term persistence is the continued survival of a critical number of virus infected cells sufficient to continue the infection without killing the host. It results from two main mechanisms:

a. Regulation of lytic potential. For viruses that do not kill their host cells, this is not usually a problem. But for lytic (virulent) viruses, there may be ways to down regulate their replicative and lytic potential so that they can persist in a state of latency without replication and damage to their host cell. This is the case with herpes viruses.

b. Evasion of immune surveillance. This may be due to several conditions that are properties of the host or the virus. Some viruses, such as influenza, can undergo antigenic shifts or antigenic drift that allows them to bypass a host immune response. Some viruses, e.g., measles, may induce a form of immune tolerance such that the host is unable to undergo an effective immune response to the virus. Other viruses, such as HIV, may set up a direct attack against cells of the immune system such that the immune system is compromised in its ability to attack or eliminate the virus.

Appendix

1.0 The Baltimore System for Virus Classification

By convention the top strand of coding DNA written in the 5' - 3' direction is + sense. mRNA sequence is also + sense. The replication strategy of the virus depends on the nature of its genome. Viruses can be classified into seven (arbitrary) groups:

I: Double-stranded DNA (Adenoviruses; Herpesviruses; Poxviruses, etc.)
Some replicate in the nucleus e.g. adenoviruses using cellular proteins. Poxviruses replicate in the cytoplasm and make their own enzymes for nucleic acid replication.

II: Single-stranded (+)sense DNA (Parvoviruses)
Replication occurs in the nucleus, involving the formation of a (-)sense strand, which serves as a template for (+)strand RNA and DNA synthesis.

III: Double-stranded RNA (Reoviruses; Birnaviruses)
These viruses have segmented genomes. Each genome segment is transcribed separately to produce monocistronic mRNAs.

IV: Single-stranded (+)sense RNA (Picornaviruses; Togaviruses, etc.)
a) Polycistronic mRNA e.g. Picornaviruses; Hepatitis A. Genome RNA = mRNA. Means naked RNA is infectious, no virion particle associated polymerase. Translation results in the formation of a polyprotein product, which is subsequently cleaved to form the mature proteins.
b) Complex Transcription e.g. Togaviruses. Two or more rounds of translation are necessary to produce the genomic RNA.

V: Single-stranded (-)sense RNA (Orthomyxoviruses, Rhabdoviruses, etc.)
Must have a virion particle RNA directed RNA polymerase.
a) Segmented e.g. Orthomyxoviruses. First step in replication is transcription of the (-)sense RNA genome by the virion RNA-dependent RNA polymerase to produce monocistronic mRNAs, which also serve as the template for genome replication.
b) Non-segmented e.g. Rhabdoviruses. Replication occurs as above and monocistronic mRNAs are produced.

VI: Single-stranded (+)sense RNA with DNA intermediate in life-cycle (Retroviruses)
Genome is (+) sense but unique among viruses in that it is diploid, and does not serve as mRNA, but as a template for reverse transcription.

VII: Double-stranded DNA with RNA intermediate (Hepadnaviruses)
This group of viruses also relies on reverse transcription, but unlike the Retroviruses, this occurs inside the virus particle on maturation. On infection of a new cell, the first event to occur is repair of the gapped genome, followed by transcription.

1.1 List of important virus families that contain genera that infect humans and the symptoms that they cause

DNA- containing viruses

Adenoviridae
        Human Adenoviruses - primarily respiratory and conjunctival infections

Astroviridae
        Astrovirus - flulike symptoms

Herpesviridae
        Herpes simplex virus type 1 - stomatitis; upper respiratory infections
        Herpes simplex virus type 2 - genital infections
        Varicella-zoster - chicken pox; herpes zoster; shingles ,
        Human Cyotmegalovirus - jaundice; hepatosplenomegaly, brain damage, death
        Epstein-Barr Virus - Burkitt's lymphoma; nasopharyngeal carcinoma; infectious mononucleosis

Papovaviridae
        Human papilloma viruses- benign tumors (warts); cervical cancer
        Human polyoma viruses - progressive leukoencephalopathy (PML); transform cells in tissue culture

Poxviridae
        Orthopoxvirus
                Variola - smallpox
                Cowpox - vesicular lesions on skin

Unclassified Round-structured viruses
        Norwalk agent "Noroviruses" - gastroenteritis

RNA - containing viruses

Arenaviridae
        Lymphocytic choriomeningitis virus (LCM) - fatal meningitis
        Lassa virus - hemorrhagic fever, frequently fatal

Bunyaviridae
         Hanta virus

Coronaviridae
        Human Coronavirus - SARS - severe acute respiratory syndrome

Filoviridae
        Ebola - acute hemorrhagic fever almost 90% case mortality
        Marburg - hemorrhagic fever, frequently fatal

Flaviviridae
        Yellow Fever - hemorrhagic fever, hepatitis, nephritis
        Dengue - fever, arthralgia, rash
        West Nile - fever, arthralgia, rash
        Hepatitis C virus - hepatitis

Orthomyxoviridae
        Influenza virus type A - acute respiratory disease
        Influenza virus type B - acute respiratory disease
        Influenza virus type C - acute respiratory disease

Paramyxoviridae
        Parainfluenza viruses - croup, common cold syndrome, mild respiratory disease
        Mumps - parotitis, orchitis, meningoencephalitis
        Measles - measles
        Subacute sclerosing panencephalitis (SSPE) - chronic degeneration of CNS
        Respiratory syncytial virus (RSV) - pneumonia and bronchiolitis in infants and children, common cold syndrome

Picornaviridae
        Human Enteroviruses
                Poliovirus - poliomyelitis
                Coxsackie virus A - aseptic meningitis, paralysis, and common cold syndrome
                Coxsackie virus B - aseptic meningitis, paralysis, severe systemic illness of newborns
                Hepatitis A virus - infectious hepatitis
                Human Rhinoviruses - common cold, bronchitis, croup, bronchopneumonia

Reoviridae
        Colorado Tick fever virus - encephalitis
        Human Rotaviruses - diarrhea in infants

Retroviridae (RNA-tumor viruses)
        Human immunodeficiency virus - acquired immune deficiency syndrome (AIDS)
        Human T-lymphotrophic virus (HTLV)

Rhabdoviridae
        Rabies virus - encephalitis, usually fatal

Togaviridae
        Eastern Equine Encephalitis virus - encephalitis
        Western Equine Encephalitis virus - encephalitis
        Rubella (Measles) - severe deformities of fetuses in first trimester of pregnancy

1.2 The Big Picture Book of Viruses provide images and links and describes viral morphology and classification www.virology.net/Big_Virology

RNA Viruses

Picornaviridae
includes enteroviruses, Hepatoviruses (hepatitis A), rhinoviruses, foot and-mouth disease virus

Togaviridae
includes rubella

Flaviviridae
includes "hepatitis C-type virus" and dengue

Retroviridae
Includes HIV, FLV, and MMTV

Paramyxoviridae
measles, mumps

Rhabdoviridae
rabies, vesicular stomatitis virus

Orthomyxoviridae
influenza

Filoviridae
Marburg and Ebola

Bunyaviridae
Hanta virus

Arenaviridae
Includes lymphocytic choriomeningitis virus

Coronavirus
SARS

DNA viruses

Adenoviridae
common cause of the common cold

Herpesviridae
includes HSV, VZV, Cyotmegalovirus and Epstein-Barr Virus

Poxviridae
smallpox, variola, cowpox (Vaccinia)

Papovaviridae
polyoma and human papilloma virus

Hepadnaviridae
Hepatitis B Virus

Parvoviridae
Canine parvovirus

Human Diseases caused by Viruses

Acute hemorrhagic conjunctivitis - Coxsackie A-24 virus (Picornavirus: Enterovirus), Enterovirus 70 (Picornavirus: Enterovirus)

Acute hemorrhagic cystitis - Adenovirus 11 and 21 (Adenovirus)

AIDS / Acquired Immune Deficiency Syndrome - human immunodeficiency virus (Retrovirus)

Bronchiolitis - Respiratory syncytial virus (Paramyxovirus), Parainfluenza virus (Paramyxovirus)

California encephalitis - California encephalitis virus (Bunyavirus)

Cervical cancer - human papilloma virus (Papovavirus)

Chickenpox - varicella zoster virus (Herpesvirus)

Colorado tick fever - Colorado tick fever virus (Reovirus)

Conjunctivitis - Herpes Simplex Virus (Herpesvirus)

Cowpox - vaccinia virus (Poxvirus)

Croup, infectious - parainfluenza viruses 1-3 (Paramyxovirus)

Dengue - dengue virus (Flavivirus)

"Devil's grip" (pleurodynia) - Coxsackie B (Picornavirus: Enterovirus)

Eastern equine encephalitis - EEE virus (Togavirus)

Ebola hemorrhagic fever - Ebola virus (Filovirus)

Gastroenteritis - Norwalk virus (Calicivirus), rotavirus (Reovirus), or various bacterial species

Genital HSV - Herpes Simplex Virus (Herpesvirus)

Gingivostomatitis - HSV-1 (Herpesvirus)

Hantavirus hemorrhagic fever / Hantaan-Korean hemorrhagic fever - Hantavirus (Bunyavirus)

Hepatitis:
Hepatitis A - hepatitis A virus (Picornavirus: Enterovirus)
Hepatitis B - hepatitis B virus (Hepadnavirus)
Hepatitis C - hepatitis C virus (Flavivirus)
Hepatitis D - hepatitis D virus (Deltavirus)
Hepatitis E - hepatitis E virus (Calicivirus)

Herpangina - Coxsackie A (Picornavirus: Enterovirus), Enterovirus 7 (Picornavirus: Enterovirus)

Herpes, genital - HSV-2 (Herpesvirus)

Herpes labialis - HSV-1 (Herpesvirus)

Herpes, neonatal - HSV-2 (Herpesvirus)

HIV - human immunodeficiency virus (Retrovirus)

Infectious myocarditis - Coxsackie B1-B5 (Picornavirus: Enterovirus)

Infectious pericarditis - Coxsackie B1-B5 (Picornavirus: Enterovirus

Influenza - Influenza viruses A, B, and C (Orthomyxovirus)

Keratoconjunctivitis - Adenovirus (Adenovirus), HSV-1 (Herpesvirus)

Lassa hemorrhagic fever - Lassavirus (Arenavirus)

Marburg hemorrhagic fever - Marburg virus (Filovirus)

Measles - rubeola virus (Paramyxovirus)

Meningitis, aseptic - Coxsackie A and B (Picornavirus: Enterovirus), Echovirus (Picornavirus: Enterovirus), lymphocytic choriomeningitis virus (Arenavirus), HSV-2 (Herpesvirus)

Mononucleosis - Epstein-Barr virus (Herpesvirus)

Mumps - mumps virus (Paramyxovirus)

Pharyngitis:
Respiratory Syncytial Virus (Paramyxovirus: Pneumovirus)
Influenza Virus (Orthomyxovirus)
Parainfluenza Virus (Paramyxovirus)
Adenovirus (Adenovirus)
Epstein-Barr Virus (Herpesvirus)

Pleurodynia - Coxsackie B (Picornavirus: Enterovirus)

Pneumonia, viral - respiratory syncytial virus (Paramyxovirus), CMV (Herpesvirus)

Polio, Poliomyelitis - Poliovirus (Picornavirus: Enterovirus)

Progressive multifocal leukencephalopathy - JC virus (Papovavirus)

Rabies - rabies virus (Rhabdovirus)

Roseola - HHV-6 (Herpesvirus)

Rubella - rubivirus (Togavirus)

Severe Acute Respiratory Syndrome (SARS) - a human coronavirus (Coronavirus)

Shingles (zoster) - varicella zoster virus (Herpesvirus)

Smallpox - variola virus (Poxvirus)

Urethritis - Herpes Simples Virus (Herpesvirus)

Varicella - varicella zoster virus (Herpesvirus)

Western equine encephalitis - WEE virus (Togavirus)

Yellow fever - Yellow fever virus (Flavivirus)

Zoster - varicella zoster virus (Herpesvirus)