What can cause an active hhv6 infection in a young man ?

Reactivation?: Almost all human herpesvirus type 6 (HHV6) infections over age two are due to reactivation of distant past childhood infection. Most humans are infected with HHV6 in early childhood -- usually without symptoms, sometimes with a skin rash illness called roseola. If the infected young man is not seriously ill, probably no worries. Otherwise discuss with the doctor who diagnosed it.