Hepatitis B virus is the DNA virus that is part of the Hepadnaviridae family. The characteristic feature of the virus from this family is that the genome consists of a circular DNA that is part double stranded and part single stranded. The virions are spherical particles with an envelope. Hepatitis B virus consists of icosahedral core within a closely adherent capsid that contains cellular lipid, glycoproteins, and a virus-specific surface antigen called Hepatitis B surface antigen (HBsAg). Respiratory syncytial virus is RNA virus that has linear single stranded nonsegmented RNA.Lassa fever virus has a circular single stranded RNA in 2 segments. Varicella-zoster virus and Epstein-Barr virus consist of a linear double stranded DNA.

Hepatitis B is an example of a chronic carrier infection where a carrier state can follow an asymptomatic infection, as well as actual disease. Neonatal rubella virus, cytomegalovirus, retrovirus, and togavirus do not cause chronic carrier infection.

Laryngotracheobronchitis, commonly known as croup, is one of the serious manifestation of parainfluenza virus infection in infants and young children. Parainfluenza type 1 is a common cause of croup, which results in subglottal swelling and airway obstruction. Major clinical manifestations include hoarseness, "barking" cough, tachypnea, tachycardia, and suprasternal retraction.

Cold sore is a recurrent mucocutaneous infection caused by herpes simplex virus (HSV). The individual retains HSV-1 DNA in the trigeminal ganglion for life following recovery from primary oropharyngeal HSV infection. Cold sores, also known as herpetic labialis, are manifested as a result of activation of HSV from the trigeminal ganglia. Cold sores are characterized by development of a cluster of vesicles around the mucocutaneous junction of the lips.

Rotaviruses can survive the acidic environment in stomach. The virus infects the small intestine and damages the epithelial cell lining the villi. The virus causes permeability changes and cell lysis, leading to malabsorption with water and electrolyte imbalance. Rotaviruses are a major cause of diarrhea in young children and are associated with outbreaks in preschool and day care-centers. Rotaviruses cause extensive tissue damage, leading to serious fluid and electrolyte loss.

Lassa fever virus is an Arenavirus associated with hemorrhagic fever. Lassa fever virus is enzootic in the West African peridomestic rodent. The rodent transmits the virus to the human by contaminating the house with urine. The infection is characterized by fever, headache, malaise, coagulopathy, petechiae, vomiting, pharyngitis, and occasional visceral hemorrhage. Serious manifestations include carditis, hepatitis, encephalopathy, pneumonitis, conjunctivitis, etc. Cardiovascular collapse results in death in 20% of the hospitalized cases.

Hand, foot, and mouth disease is a vesicular exanthem caused by coxsackievirus A16. It is characterized by ulcerating vesicles on hand, foot, mouth, and tongue along with mild fever. The disease subsides in a few days on its own.

Parvovirus B19 is a member of the family Parvoviridae and is the causative agent of erythema infectiosum (fifth disease). The hallmark symptom of erythema infectiosum is a mild rubella-like erythematous rash that produces arthropathy, especially in women. The disease has two separate components. The first component is the lytic phase where the lytic action of the virus replication in erythroid precursors gives rise to a transient arrest of erythrocytic production. This can result in a slight drop in hemoglobin. Hemoglobin levels will substantially drop in those patients with chronic hemolytic anemia. This is the viremic stage, and mild flu-like symptoms with pyrexia and chills are not uncommon. These flu-like symptoms can last for 1 to 3 days. One week after the viremia peak, the second phase occurs where the virus is rarely detectable. The second phase is where the erythematous maculopapular rash and arthropathy occur. The characteristic rash produces a lace-like appearance on the trunk and extremities and a “slapped-cheek” appearance on the face. Erythema infectiosum infections in pregnant women can induce fetal loss (5-20%), especially if the infection occurs at less than 20 weeks.

Herpes simplex type 1 (HSV-type 1) belongs to the family Herpesviridae and causes an infection that characteristically presents as an eruption of vesicles that are restricted to areas around the mouth, lips, and nostrils. The infection can also occur in the genital area, thought HSV-type 2 is more commonly associated with genital infections, called herpes labialis. Patients usually can predict the onset of an eruption due to a prodromal stage of itching or other sensations that can occur a few hours to a few days prior to the eruption of vesicles. HSV-type 1 also causes primary infections of the oral mucous membranes in children and herpetic eye infections. Eruptions can occur at any time and depend on the immunological status of the host, health, and various other factors that vary from host to host.

Herpes simplex type 2 (HSV-type 2) belongs to the family Herpesviridae and causes an infection that characteristically presents as an eruption of vesicles that are restricted to areas around the genital area. Multiple vesicles in females appear on the vulva, in the vagina, and on the portio. Multiple vesicles in men are found often on the sulcus of the glans and the preputium but can be found on any part of the penis. Duration and severity of the eruptions are usually more severe in females than in males. HSV-type 1 can also be found to cause genital eruptions but at a significantly lower rate. HSV-type 2 causes 75% of all neonatal herpes (a severe complication of genital HSV infection) where the newborn acquires the virus by contamination through the birth canal. Eruptions can occur at any time and depend on the immunological status of the host, health, and various other factors that vary from host to host.

Epstein-Barr virus belongs to the family Herpesviridae and is the causative agent of infectious mononucleosis (IM), Burkitt's Lymphoma, and undifferentiated nasopharyngeal carcinoma. IM characteristically produces a marked lymphocytosis involving almost all of the lymphoreticular tissues. This leads to hepatomegaly, splenomegaly, lymphadenopathy, and lymphoid hyperplasia of the oronasopharynx. The peripheral blood smear contains atypical lymphocytes of the Downey II type. It is generally a self-limiting disease; however, it can cause a severe protracted illness that can at times prove to be fatal. Patients with an unusual immuno-defect known as the X-linked lymphoproliferative syndrome frequently have overwhelming EBV primary infections, which can be fatal in the majority of the cases.

Cytomegalovirus (CMV) belongs to the family Herpesviridae and causes cytomegalovirus mononucleosis, cytomegalic inclusion disease, and cytomegalovirus infection in the immunocompromised. CMV resembles infectious mononucleosis in that atypical lymphocytosis, fever, and malaise are characteristic symptoms to both. Pharyngitis is present as a symptom, though less severe in CMV than in infectious mononucleosis. The heterophile antibody test is also negative in CMV. Toxoplasmosis may also mimic CMV and infectious mononucleosis and thus must be considered in the differential diagnosis in those patients with a negative heterophile antibody test and mono-like symptoms. Transmission of CMV from mother to fetus may occur at any time during the pregnancy.  Fetal loss is less likely than congenital CMV infection, which – in its most severe form – is manifested as hepatosplenomegaly, microcephaly, chorioretinitis, hearing loss, and thrombocytopenia.  The mortality rate is high.  

Human papillomaviruses are small DNA viruses. These viruses replicate in the nuclei of differentiated keratinocytes and are the cause of tumors of the skin and mucosa. The genome of human papillomaviruses is circular and contains approximately 8,000 base pairs.

No suitable cell culture system is currently available for the hepatitis B virus. Fortunately, the hepatitis B virus produces a DNA-dependent polymerase that is virus specific. Influenza A, influenza B, mumps virus, and parainfluenza virus may be cultured in primary monkey kidney cells. Influenza A, B, and C all may be cultured in eggs. Influenza A and B may be detected by hemadsorption, cytopathic effect, and hemagglutination; Influenza C may be detected by hemagglutination. Retroviridae may be cultured in T lymphocytes with methods of detection of transformation, reverse transcriptase, cytopathic effect, or immunofluorescence.

Human papillomavirus, herpes simplex virus, and hepatitis B virus are a sexually-transmitted viruses with double-stranded DNA genomes. While herpes simplex virus has a relatively large genome, the genome of both human papillomavirus and hepatitis B virus are both quite small.

The Tzanck smear stains for the presence of multinucleated giant cells that are formed as a result of fusion of cell membranes due to insertion of viral proteins into the membrane. Tzanck smear does not detect the presence of inclusion bodies, dead cells, herpes DNA, or cell envelope.

Herpes simplex type 1 and 2 (HSV-1, HSV-2) are both large-enveloped DNA viruses which cause latent and recurrent infections. However, HSV-1 is primarily associated with infections of the pharynx, skin, and central nervous system. On the other hand, HSV-2 is primarily associated with genital infections, representing approximately 70% of all such initial infections. HSV-2 infections recur more often as well. Human immunodeficiency virus type 1 and 2 (HIV-1 and HIV-2) are retroviruses with plus-strand RNA genomes. Both of these can be passed by sexual contact and result in long-term infection of multiple cells and tissues. Both can also give rise to acquired immune deficiency syndrome or AIDS, however HIV-1 is associated with a long-term infection which has been virtually 99% fatal after 10-15 years. Recent research has illustrated that a small number of individuals may be infected with somewhat less virulent strains and these individuals do not follow the normal time-course to AIDS. HIV-2 is associated with a far less virulent infection and may include a much larger percentage of "none-progressing" individuals as well. 

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:

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

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

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