A Roseola is a common, harmless viral infection to which babies seem to be most susceptible. It's so typical, actually, that during the routine 6-month checkup, I often mention it to parents so they'll be prepared if their baby gets it. Roseola is characterized by a faint, rose-pink rash that appears first on the chest, then spreads to the face, neck, and shoulders. It's much less intense than viral illnesses like measles -- it can be so light that you may not even notice it -- and it usually vanishes after a day. A few days before the rash, the baby will have a high temperature (103°F to 105°F) but in most cases no other symptoms. The fever lasts for several days and then disappears as suddenly as it began. Babies are most contagious the day before they get the fever and during it. So if your child's infected, let the parents of kids who were exposed to him at that time know. That way, they won't be alarmed if their children develop a fever in a week or two.
Since the rash appears after the fever's gone, roseola can be tricky to diagnose. It's best for doctors to avoid prescribing antibiotics at the onset of a fever, since viruses don't respond to them, and if the medication is given and the baby does have roseola, parents may worry that the rash is an allergic reaction.
Aside from treating the high fever with acetaminophen or ibuprofen (for babies over 6 months) if your child seems uncomfortable, you'll likely need to wait the virus out -- while continuing to feed and dress him as you normally would.