Viral Infections

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.

EBV can be cultured from the saliva of 10-20% of healthy adults. Excretion may persist weeks to months. Infection with EBV is by contact with infected secretions such as saliva. It is of low contagiousness, and most cases of infectious mononucleosis are contracted after repeated contact between susceptible persons and those asymptomatically shedding the virus.

CMV is ubiquitous, and in developed countries ~50% of adults have developed antibody (Box 7). Age-specific prevalence rates show that ~ 10-15% of children are infected by CMV during the first 5 years of life, after which the rate of new infections levels off. The rate subsequently increases during young adulthood, probably through close personal contact or sexual transmission of the virus.

VZV infection, the cause of both varicella (chickenpox) and herpes zoster, is ubiquitous (Box 4). Nearly all persons contract chickenpox before adulthood, and 90% of cases occur before the age of 10. The virus is highly contagious, with attack rates among susceptible contacts of 75%. Varicella occurs most frequently during the winter and spring months.

The term herpes (from the Greek herpein, to creep) and the clinical description of cold sores date back to Hippocrates. Two distinct epidemiologic and antigenic types of HSVs exist (HSV-1 and HSV-2). HSVs have worldwide distribution.

Adenoviruses were first isolated in 1953 in human adenoid cell culture. Since then approximately 100 serotypes, at least 47 of which infect humans, have been recognized. All human serotypes are included in a single genus within the family Adenoviridae. Based on homology studies and hemagglutination patterns, each of the 47 serotypes belongs to one of six subgroups.