Roseola is a contagious disease caused by a virus that initially causes a high fever in children who are infected. Once the fever breaks, a skin rash develops first on the trunk and then it spreads over the body.

Children from the ages of 6 months to 3 years are most vulnerable to roseola, although the most common time for it to occur is from 6 to 12 months of age. Also known as exanthem subitum, the illness is typically mild after the initial high fever and most children make a quick and full recovery. In some cases, the high fever associated with the condition triggers seizures. Adults rarely develop the disease but may transmit the infection to children. 
 

Roseola is extremely common. After a child is exposed to the virus that causes roseola, it may take about five to 15 days before symptoms appear. In some cases, children who contract the virus never display any symptoms.

Drops of fluid from the nose and throat of infected adults or children can transmit roseola to others when the person talks, laughs, sneezes or coughs. Transmission occurs through breathing in germs or touching the nose or mouth after a finger has been exposed to a germ. Although roseola is contagious, it rarely causes the kind of widespread outbreaks associated with conditions such chickenpox. It appears that children who have symptoms of roseola no longer are contagious.

Roseola occurs year-round but most cases develop in the spring and fall. Children who recover from roseola appear to develop some immunity to the disease, and it is rare for a child to develop roseola more than once. By the time most people are of school age, they have developed antibodies to roseola and are immune to infection. However, adults occasionally do become infected. In such cases, the disease tends to be milder than it is in children.

Two closely related viruses cause roseola – human herpesvirus type 6 and type 7. The herpesvirus family consists of a large number of viruses, including the one that causes chickenpox and shingles (varicella zoster virus). These viruses are also related to herpes simplex viruses, but do not cause the cold sores or genital sores common to those forms of virus. Human herpesvirus type 6 is the most common cause of roseola. 

Roseola is most likely to strike older infants because they have not yet developed antibodies against many types of viruses, and have lost the antibodies they received from their mother while in the womb.

In addition, patients with weakened immune systems, such as those who have received a recent bone marrow or organ transplant, are more likely to develop roseola and to have a previous infection return. They are also more likely to develop a serious case of roseola, have a more difficult time fighting the condition, and are more likely to develop complications (e.g., pneumonia, encephalitis).