The epidemiologic picture of human herpesvirus 6 (HHV6) has continually evolved over its known life span of 2 decades. Still, it remains more of a collage than a completed portrait, for HHV6 has multiple characters. It is a covert companion of life, and yet a mimicker of many maladies.
Although "HHV6" was named relatively recently, this virus probably has long been recognized as an exanthematous disease of childhood. In papers and texts from the 1800s, an acute infection of young children with rash was given such sobriquets as roseola infantilis, exanthem criticum, exanthem subitum, the rash of roses, and prophetically, the sixth exanthematous disease of childhood. These old descriptions suggest that even in ages past infections likely from HHV6 were common, geographically widespread, and occurred in young children, primarily infants.
Zahorsky's (1913) observations in the early 1900s on 44 cases of roseola and Breese's (1941) subsequent prospective study in 1941 of 111 cases confirmed the disease as an entity with distinctive characteristics, even in the pre-eruptive phase. Breese made a number of clinical and epidemiologic observations that remain true today. He noted that 16% of the infants he followed closely from the time of birth to 1 year of age developed clinical roseola, and 30% by 3 years of age. The peak occurrence (12-13%) was in the 10th month of life, and the illness was mostly limited to children between the ages of 6 months and 3 years of age. The children had no contact with similar cases, and most were exposed only to healthy family members. To determine the cause and mode of transmission, Breese attempted to isolate a ''filterable virus'' from the infant's respiratory and blood specimens.
Kempe and co-workers (1950) subsequently also attempted unsuccessfully to isolate the virus. They did, however, show that the infectious agent was present in serum and throat washings. Kempe et al. (1950) further speculated that roseola might have ''been a cause of epidemic disease centuries ago and to which constitutional immunity is present in adults. Among these adults the agent frequently could be present as a latent infection and susceptibility to the pathogen might exist only in the immuno-logically immature'', and he also suggested herpes simplex virus as a possibility.
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