Part IV: Clinical Virology: Introduction

For medical purposes, viruses are most usefully described in terms of either their main site of infection, their mode of transmission, or the type of lesions and diseases they cause. Chapters 37 to 45 describe the clinically important viruses organized in this medically relevant manner. Several less prominent viruses are described in Chapter 46.

In this brief introduction, the clinically important viral pathogens are categorized into groups according to their main structural characteristics (i.e., DNA enveloped viruses, DNA nonenveloped¹ viruses, RNA enveloped viruses, and RNA nonenveloped viruses) (see Table IV–1).

Herpesviruses

These viruses are noted for their ability to cause latent infections. This family includes (1) herpes simplex virus types 1 and 2, which cause painful vesicles on the face and genitals, respectively; (2) varicella-zoster virus, which causes varicella (chickenpox) typically in children and, when it recurs, zoster (shingles); (3) cytomegalovirus, an important cause of congenital malformations; (4) Epstein–Barr virus, which causes infectious mononucleosis; and (5) human herpesvirus 8, which causes Kaposi’s sarcoma. (See Chapter 37.)

Hepatitis B Virus

This virus is one of the important causes of viral hepatitis. In contrast to hepatitis A virus (an RNA nucleocapsid virus), hepatitis B virus causes a more severe form of hepatitis, results more frequently in a chronic carrier state, and is implicated in the induction of hepatocellular carcinoma, the most common cancer worldwide. (See Chapter 41.)

Poxviruses

Poxviruses are the largest and most complex of the viruses. The disease smallpox has been eradicated by effective use of the vaccine. Molluscum contagiosum virus is the only poxvirus that causes human disease in the United States at this time. (See Chapter 37.)

Adenoviruses

These viruses are best known for causing upper and lower respiratory tract infections, including pharyngitis and pneumonia. (See Chapter 38.)

Papillomaviruses

These viruses cause papillomas on the skin and mucous membranes of many areas of the body. Some types are implicated as a cause of cancer (e.g., carcinoma of the cervix). (See Chapter 37.)

Parvovirus B19

This virus causes “slapped cheeks” syndrome, hydrops fetalis, and severe anemia, especially in those with hereditary anemias such as sickle cell anemia. (See Chapter 39.)

Respiratory Viruses

1. Influenza A and B viruses. Influenza A virus is the major cause of recurrent epidemics of influenza. (See Chapter 38.)

2. Parainfluenza viruses. These viruses are the leading cause of croup in young children and an important cause of common colds in adults. (See Chapter 38.)

3. Respiratory syncytial virus. This virus is the leading cause of bronchiolitis and pneumonia in infants. (See Chapter 38.)

4. Human metapneumovirus. This virus causes significant upper and lower respiratory tract disease. (See Chapter 38.)

Measles, Mumps, and Rubella Viruses

These viruses cause well-known childhood diseases and are the viral components of the MMR vaccine. Widespread use of the vaccine has markedly reduced the incidence of these diseases in the United States. These viruses are well known for the complications associated with the diseases they cause (e.g., rubella virus infection in a pregnant woman can cause congenital malformations). (See Chapter 39.)

Rabies Virus

This virus causes almost invariably fatal encephalitis following the bite of a rabid animal. In the United States, wild animals such as skunks, foxes, raccoons, and bats are the major sources, but human infection is rare. (See Chapter 39.)

Hepatitis C Virus

This virus causes hepatitis C, the most prevalent form of viral hepatitis in the United States. It causes a very high rate of chronic carriers and predisposes to chronic hepatitis and hepatic carcinoma. (See Chapter 41.)

Human T-Cell Lymphotropic Virus

This virus causes T-cell leukemia in humans. It also causes an autoimmune disease called tropical spastic paraparesis. (See Chapter 43.)

Human Immunodeficiency Virus

Human immunodeficiency virus (HIV) causes acquired immunodeficiency syndrome (AIDS). (See Chapter 45.)

Enteroviruses

These viruses infect the enteric tract and are transmitted by the fecal–oral route. Poliovirus rarely causes disease in the United States because of the vaccine but remains an important cause of aseptic meningitis and paralysis in developing countries. Of more importance in the United States are Coxsackie viruses, which cause aseptic meningitis, myocarditis, and pleurodynia; and echoviruses, which cause aseptic meningitis. (See Chapter 40.)

Rhinoviruses

These viruses are the most common cause of the common cold. They have a large number of antigenic types, which may account for their ability to cause disease so frequently. (See Chapter 38.)

Rotaviruses

These viruses possess an unusual genome composed of double-stranded RNA in 11 segments. Rotaviruses are an important cause of viral gastroenteritis in young children. (See Chapter 40.)

Hepatitis A Virus

This virus is an important cause of hepatitis. It is an enterovirus but is described in this book in conjunction with hepatitis B virus. It is structurally different from hepatitis B virus, which is a DNA enveloped virus. Furthermore, it is epidemiologically distinct (i.e., it primarily affects children, is transmitted by the fecal–oral route, and rarely causes a prolonged carrier state). (See Chapter 41.)

Noroviruses

Noroviruses are a common cause of gastroenteritis, especially in adults. They are a well-known cause of outbreaks of vomiting and diarrhea in hospitals, in nursing homes, and on cruise ships (See Chapter 40).

Hepeviruses

The main human pathogen in the hepevirus family is hepatitis E virus (HEV). It causes hepatitis acquired by fecal–oral transmission similar to hepatitis A virus. HEV is a nonenveloped virus with a positive-polarity single-stranded RNA genome. (See Chapter 41.)

Chapter 42 describes the large and varied group of arboviruses, which have the common feature of being transmitted by an arthropod. Chapter 43 discusses tumor viruses, and Chapter 44 covers the “slow” viruses that cause degenerative central nervous system diseases primarily. Chapter 45 describes HIV, the cause of AIDS. The less common viral pathogens are described in Chapter 46.