Also called exanthem subitum, this is a very common childhood infection that is prevelent in the 6mth - 2yr age group. It's caused by the human herpes virus type 6 (HHV-6), and it infects the CD4+ T lymphocytes. It occurs world-wide, and is often a latent infection in adults, with 85% of the population testing positive for it.
In children, it causes a sudden onset fever, making the child very irritable. It's temperature may rise very high, to 40.5oC. This will continue for four or five days before suddenly falling back down again. At this time, a rash will appear on the child's trunk, before spreading to the limbs, neck and face. This rarely lasts longer than a day or two. The child may also complain of a sore throat, and swollen lymph glands may be found in the neck.
Treatment for roseola infantum is supportive, keeping the child cool and giving it some paracetamol if necessary to help reduce the fever. If the child is really uncomfortable and irritable then it may help to cool it down a little by sponging with tepid water.If the child is immunocompromised, then doctors may prescribe a anti-viral drug such as ganciclovir.
Possible complications of the disease include febrile convulsions, and, very rarely, an aseptic meningitis or encephalitis may occur.
Thanks to doyle for helping by correcting some of this!