Hepatitis C is an enveloped icosahedral RNA virus with gene order characteristic of the family Flaviviridae. The genome of Hepatitis C is infectious and consists of a linear single stranded 9.5 kb molecule of RNA. The virus replicates in the cytoplasm. Hepatitis C is transmitted by parenteral and sexual routes. It is the most common cause of post-transfusion hepatitis. Hepatitis C is associated with acute infection, chronic liver disease, and cirrhosis. Hepatitis C viral infection may serve as a predisposing factor in primary hepatocellular carcinoma.

Hepatitis B virus is an enveloped DNA virus that is part of the Hepadnaviridae family. The genome of Hepadnaviridae family is characterized by the presence of a circular, partially double stranded DNA molecule. Hepatitis B virus consists of a icosahedral core within a closely adherent capsid that contains cellular lipid, glycoproteins, and a virus-specific surface antigen called Hepatitis B surface antigen (HBsAg). The virus encodes a reverse transcriptase and replicates through an RNA intermediate. Hepatitis B virus shows tropism to the liver and replicates in hepatocytes. Hepatitis B is also called serum hepatitis and is caused by a DNA virus known as hepatitis B virus. It is transmitted by parenteral or sexual routes. Hepatitis B has a longer incubation period, which is approximately 50-160 days. Onset of the disease is gradual and the infection can be acute or self-limiting or can lead to persistent viremia and chronic liver disease with immunologic complications.

The Hepatitis A virus is a naked icosahedron that is member of the Picornaviridae family. The genome of this virus is infectious and contains a single linear molecule of single stranded RNA. Hepatitis A is transmitted by feco-oral routes and was known as infectious hepatitis. The virus enters the body by ingestion and multiplies in intestinal epithelial cells. The virus then enters the circulation and invades parenchymal cells in the liver. Incubation period for Hepatitis A is 15-40 days.

Hepatitis D virus is a very small virus with tiny genome and is classified under genus Deltavirus. It is defective satellite virus and needs Hepatitis B virus as helper, i.e., it is infective only in the presence of Hepatitis B virus. The outer capsid of the virion contains Hepatitis B surface antigen, which is encoded by HBV co-infecting the same cell.

Hepatitis E virus is a naked virus, which has icosahedral capsid with surface depression. Hepatitis E virus is placed in a sole genus Hepevirus, within a new family Hepeviridae, and contains a single molecule of a single stranded RNA. Hepatitis A is transmitted by feco-oral routes and by contaminated food and water. Hepatitis E is associated with cholestasis and has a higher fatality rate in pregnant women.

Hepatitis A virus and Hepatitis E virus are not associated with the following:

• Parenteral transmission
• Chronic carrier state
• Chronic hepatitis
• Cirrhosis
• Hepatocellular carcinoma

Hepatitis B virus, Hepatitis C virus, and Hepatitis D virus are associated with the following:

• Parenteral transmission
• Chronic carrier state
• Chronic hepatitis
• Cirrhosis
• Hepatocellular carcinoma

Among the respiratory viruses that can be isolated from throat secretions are influenza, parainfluenza, respiratory syncytial virus, and rhinovirus. These may also be present in nasal washes, nasopharyngeal swabs, lung biopsy, and aspirates of the lower respiratory tract. Additional respiratory viruses, which may be isolated from the stool in addition to the aforementioned locations, are the enterovirus and adenovirus. Certain viruses, which affect the central nervous system, may be isolated from throat swabs, including enterovirus. Cytomegalovirus and herpes simplex viruses are among the viruses, which also can cause CNS illness. These two also cause congenital or perinatal exanthem, in which case isolation may be made from throat swabs.

Prions are infectious particles composed solely of proteins. They are not composed of viral nucleic acids and do not need a helper virus to replicate. They do not contain host cell DNA instead of viral DNA within the capsid; they do not consist solely of a single molecule of circular RNA without a protein coat or envelope; and they are not protein particles that generate a strong immune response.

Hairy leukoplakia may be seen in people infected with HIV and AIDS. It is characterized by raised areas that are whitish-tan in color that have a feathery appearance. It is different from candidiasis of the tongue in that hairy leukoplakia cannot be scraped off. A 'hairy tongue' is not actually due to hair growth on the tongue; it consists of elongated papillae that have the appearance of grayish-black hair to the naked eye. This condition may be caused by antibiotic use, or there may not be any reason. Atrophic glossitis (or smooth tongue) presents as having a smooth surface due to papillae loss. The loss may indicate deficiency in riboflavin, niacin, folic acid, vitamin B12, pyridoxine, or iron. A geographic tongue is a benign condition with an unknown cause; it is characterized by a map-like pattern of smooth, red areas that do not have papillae as well as rough areas that still have papillae. Although candidiasis is also common with people with AIDS and the infection also causes the tongue to have a white coating, the coat can be scraped, and a sample can be analyzed for the presence of Candida.

A retrovirus contains RNA as its genetic material, and uses the enzyme reverse transcriptase to convert its single stranded RNA genome into a DNA/RNA helix which is then converted to a DNA duplex. This then inserts into the host chromosome, is replicated, and eventually gives rise to single stranded RNA to be incorporated into the new viral particle. RNA and DNA have three bases in common (adenine, guanine, and cytosine) but RNA contains uracil and DNA contains thymine as the fourth base. Therefore, finding uracil in the viral genome indicates that it is an RNA containing virus. Since it contains reverse transcriptase, we can deduce that it is retrovirus.

A prion is a type of infectious agent that lacks nucleic acid, and is made up only of protein. Prion is a short form for proteinaceous infectious particle. The process of incorporating the genetic material of a virus into the host genome is known as integration. Retroviruses can undergo integration. Bacteriophages (and not retroviruses) infect bacteria only.

The keys to this question are the terms "universally useful" and "infective." Some of the described methods lead to the counting of both viable and nonviable particles, as described below.

Electron microscopy is useful for enumerating purified virus particles present at high density. The addition of a known number of latex particles to the suspension, followed by the calculation of the ratio of those particles to viral particles, provides an accurate count. Of course, this technique has the disadvantage that viable and nonviable particles cannot be differentiated. Equally important, very few diagnostic laboratories, and indeed not all research laboratories, have ready access to an electron microscope.

The plaque assay is the most precise measure of infectivity. The basis of this technique is the formation of clear zones, plaques or foci, in a monolayer of host cells. Serial dilutions of the virus are added to a monolayer of cells known to be infected by the cells (for example, MDCK cells are used for influenza). Most commonly after the virus has been allowed to adsorb onto the cells, an agar overlay is added to prevent dispersion of the virus. This ensures that each plaque represents only a single initial virus particle. For viruses which only infect contiguous cells without releasing virions into the surrounding medium, the agar may be replaced by a liquid.

Certainly not applicable for all viruses, the pock assay involves the counting of localized lesions (pocks) that form on the chorioallantoic membrane (CAM) of the embryonated egg upon addition of known dilutions of the virus. As in the plaque assay, each lesion represents a single original virus particle.

Instead of providing a quantitative measure of viral infectivity, quantal assays provide only a "yes or no" result. In quantal assays, each viral replication cycle results in an amplification of the effects of each individual virus particle. The virus is serially diluted, and appropriate cells are incubated with a known volume of each dilution. The endpoint for these assays may be negative effects, including morbidity in animals and development of lesions on either the chorioallantoic membrane or on the embryo of eggs. Following the incubation period, each target animal, or sample, is examined for a +/- response.

Epstein-Barr virus and cytomegalovirus elicit a large T-cell response, resulting in a mononucleosis-like syndrome. EBV infection is subclinical and milder in children than adolescents or adults. EBV initiates infection in the epithelial cells of the oropharynx and then spreads to the B lymphocytes in lymphatic tissue and blood. Classical lymphocytosis is associated with the activation and proliferation of suppressor T cells, and it leads to infectious mononucleosis. The atypical lymphocyte, also known as "Downey cells", increases in peripheral blood during the second week of infection. Major T-cell response causes swollen lymph glands, spleen, and liver. Fever, malaise, fatigue, pharyngitis, lymphadenopathy, and hepatosplenomegaly are the classical symptoms associated with infectious mononucleosis.

Cytomegalovirus is associated with heterophile-negative mononucleosis. Polyclonal B cell activation by EBV leads to production of heterophile antibodies, which can be detected by the end of first week of infection and last for several months.

Respiratory syncytial virus, varicella zoster virus, and influenza virus are not associated with infectious mononucleosis.

Except for the Reoviridae, which contains the orbiviruses, rotaviruses, and reoviruses, all of the families mentioned are DNA viruses. The Parvoviridae and Adenoviridae families contain the Parvovirus and Adenovirus, respectively. The viruses in both of these families have a naked virion (i.e., without an envelope), but viruses in the former family have a complex nucleocapsid structure, while those in the latter family have an icosahedral nucleocapsid symmetry. The herpes simplex virus, cytomegalovirus, Varicella -Zoster virus, and the Epstein-Barr virus all belong to the Herpesviridae family. The family of Poxviridae contains complex viruses with a naked virion, such as the vaccinia, smallpox, and variola viruses.

Rickettsia, chlamydia, prions, and viroids can be distinguished from viruses, but not by just one of the mentioned criteria. While all of these groups are obligate intracellular organisms, only the viruses are parasites with an absolute intracellular requirement, which also depend on the machinery of the host cell for replication. Prions are small infectious protein particles. Viroids are subviral infectious nucleic acids, which lack even the protein capsid found in viruses. Viruses may contain a genome of DNA or RNA, so the presence of an RNA genome is indicative of a virus, but the presence of a DNA genome is not.

Hepatitis C is an enveloped icosahedral RNA virus with gene order characteristic of the family Flaviviridae. The genome of Hepatitis C is infectious and consists of a linear single stranded 9.5 kb molecule of RNA. The virus replicates in the cytoplasm. Hepatitis C is transmitted by paranteral and sexual routes. It is the most common cause of post-transfusion hepatitis. Hepatitis C is associated with acute infection, chronic liver disease, and cirrhosis. Hepatitis C viral infection may serve as a predisposing factor in primary hepatocellular carcinoma.

Hepatitis B virus is an enveloped DNA virus that is part of the Hepadnaviridae family. The genome of Hepadnaviridae family is characterized by the presence of a circular, partially double stranded DNA molecule. Hepatitis B virus consists of a icosahedral core within a closely adherent capsid that contains cellular lipid, glycoproteins, and a virus-specific surface antigen called Hepatitis B surface antigen (HBsAg). The virus encodes a reverse transcriptase and replicates through an RNA intermediate. Hepatitis B virus shows tropism to the liver and replicates in hepatocytes. Hepatitis B is also called serum hepatitis and is caused by a DNA virus known as hepatitis B virus. It is transmitted by parenteral or sexual routes. Hepatitis B has a longer incubation period, which is approximately 50-160 days. Onset of the disease is gradual and the infection can be acute or self-limiting or can lead to persistent viremia and chronic liver disease with immunologic complications.

The Hepatitis A virus is a naked icosahedron that is member of the Picornaviridae family. The genome of this virus is infectious and contains a single linear molecule of single stranded RNA. Hepatitis A is transmitted by feco-oral routes and was known as infectious hepatitis. The virus enters the body by ingestion and multiplies in intestinal epithelial cells. The virus then enters the circulation and invades parenchymal cells in the liver. Incubation period for Hepatitis A is 15-40 days.

Hepatitis D virus is a very small virus with tiny genome and is classified under genus Deltavirus. It is defective satellite virus and needs Hepatitis B virus as helper, i.e., it is infective only in the presence of Hepatitis B virus. The outer capsid of the virion contains Hepatitis B surface antigen, which is encoded by HBV co-infecting the same cell.

Hepatitis E virus is a naked virus, which has icosahedral capsid with surface depression. Hepatitis E virus is from the family Calciviridae and contains a single molecule of a single stranded RNA. Hepatitis A is transmitted by feco-oral routes and by contaminated food and water. Hepatitis E is associated with cholestasis and has a higher fatality rate in pregnant women.

Hepatitis A virus and Hepatitis E virus are not associated with the following:

• Parenteral transmission
• Chronic carrier state
• Chronic hepatitis
• Cirrhosis
• Hepatocellular carcinoma

Hepatitis B virus, Hepatitis C virus, and Hepatitis D virus are associated with the following:

• Parenteral transmission
• Chronic carrier state
• Chronic hepatitis
• Cirrhosis
• Hepatocellular carcinoma