Roseola has been around for centuries and has many names to prove it: roseola infantum, exanthema subitum, three-day fever, and sixth disease. It is the sixth in a list of classic childhood illnesses involving a fever and rash that medical students memorize long before they see them in the flesh. Yet unlike measles, rubella, and scarlet fever, the agent causing roseola was unknown to medicine until just recently, when it was discovered that a virus called human herpes virus 6 (HHV6) is responsible for this ubiquitous childhood illness. With this knowledge, the medical community hopes to find accurate diagnostic tests for this germ, and perhaps an effective medicine to treat it.
A Routine Infection
Infection with HHV6 is so common that antibodies against it can be found in 97 percent of blood donors, meaning almost all adults have at some point been infected with this virus. Roseola is but one manifestation of HHV6 infection, and about 30 percent of children between six months and two years will suffer from it. Yet 90 percent of one-year-olds will show antibodies against HHV6 in their blood, meaning that many, many infections are either without symptoms or present as a vague febrile illness during infancy.
For a child’s first six months of life, antibodies from the mother protect the infant from infection. After this age, infection rates skyrocket—making roseola almost exclusively an infection of infancy and toddlerhood. Like all viruses in the herpes family (varicella causing chicken pox, herpes simplex causing genital or oral herpes), HHV6 causes an initial infection which resolves, but the virus is never cleared from the body. It remains latent within a body and has the potential to reactivate years, even decades later. In the case of varicella, this means shingles, and for herpes simplex, it is recurrent genital lesions or cold sores. It is unclear what a reactivation of HHV6 in healthy adults means, but a reactivation in an immuno-compromised person can cause serious illness. This virus is only found in humans, and at any one time 75 percent of healthy adults are shedding live, infectious virus into their saliva. A kiss or close contact is all it takes for an adult to unknowingly spread HHV6 to an infant.