Author: Brian Holtry

Delta hepatitis is spread just as hepatitis B and is most prevalent in groups at high risk of hepatitis B infection. Injection drug users are those at greatest risk in the western world, and ~ 50% of such individuals may have IgG antibody to the delta virus antigen. Delta virus infection is rare in the U. S., northern Europe, and Japan but largely prevalent in southern Europe, Africa, and South America.

Hepatitis B infection is found worldwide, with prevalence rates varying markedly among countries. Chronic carriers constitute the main reservoir of infection: in some countries, particularly in the Far East, ~ 5-15% of all persons carry the virus, though most are asymptomatic. Of patients with HIV infection, 10% are chronic carriers of hepatitis B.

Hepatitis A virus is the cause of what was formerly termed infectious hepatitis or short-incubation hepatitis. It was first detected in the early 1970s in stools of patients incubating the disease. Humans appear to be the major natural hosts of hepatitis A virus.

Even in developed countries, antibodies to caliciviruses are nearly universal by age 5. Infections tend to occur in family or community outbreaks. In underdeveloped countries, infection by these viruses occurs presumably as a result of poor sanitation. In developed countries, outbreaks that occur year-round have been described in schools, resorts, hospitals, nursing homes, restaurants, and cruise ships.

Rotavirus is the most common etiology of acute diarrheal illness in children < 2 years old. It is responsible for > 1 million cases of reported diarrheal illness each year in children ages 1-4 years in the United States. Rotavirus is responsible for an average of 150 deaths per year in this same group. All of these deaths are secondary to severe dehydration.

The epidemiology of rubella, commonly referred to as German measles or 3-day measles, has changed dramatically in the past 30 years, owing exclusively to the widespread use of the rubella live attenuated virus vaccine. Before the use of this vaccination (1969), the virus had an epidemic cycle of 6-9 years.

Mumps, historically known as epidemic parotitis, was one of the most common early childhood infections before the routine use of mumps vaccination starting in 1968. Reported cases of mumps have dropped 98% when compared with the prevaccine era. It is spread primarily during the late winter and early spring. Before the vaccination era, mumps epidemics occurred in 3- to 4-year cycles.

Rubeola, commonly known as measles, is a virus spread primarily in the winter and early spring. Like mumps and rubella, vaccination has drastically changed the epidemiology of measles.

The herpesvirus group of the family Herpesviridae comprises large, enveloped, double-stranded DNA viruses found in both animals and humans. They are ubiquitous and produce infections ranging from painful skin ulcers to chickenpox to encephalitis. The major members of the group to infect humans are the two herpes simplex viruses (HSV-1 and -2), cytomegalovirus (CMV), varicella-zoster virus (VZV), Epstein-Barr virus (EBV), herpesvirus 6, and the recently discovered human herpesvirus types 7 and 8. Occasionally, the simian herpesvirus, herpes B virus, has caused human disease.

In 1986 a human herpesvirus, now called human herpesvirus type 6 (HHV-6), was identified in cultures of peripheral blood lymphocytes from patients with lymphoproliferative diseases (Box 2). The virus, which is genetically distinct but morphologically similar to other herpesviruses, replicates in lymphoid tissue, especially CD4+ T lymphocytes, and has two distinct variants, A and B. Initially it was thought that HHV-6 would grow only in freshly isolated B-lymphocytes, and the virus was referred to as the human B-lymphotropic virus (HBLV); now it is clear that the virus is preferentially tropic for CD4+ T lymphocytes. HHV-6 establishes a latent infection in T cells but may be activated to a productive lytic infection by mitogenic stimulation.