How do people get roseola?

Virus. Roseola is a viral infection that causes a rash and fever. It is common in infants and children. The most common virus is human herpesvirus 6, but other viruses can cause the same clinical picture. Not the same herpes virus that causes painful genital blisters. It is spread from contact with respiratory secretions (ie saliva) of an infected person (usually before symptoms even show it is spread).