Exanthem subitum (meaning sudden rash), also referred to as roseola infantum (or rose rash of infants), sixth disease and (confusingly) baby measles, is a benign disease of children, generally under two years old, whose manifestations are usually limited to a transient rash ("exanthum") that occurs following a fever of about three day's duration. Until recently, its cause was unknown: it is now known to be caused by two human herpesviruses, HHV-6 and HHV-7, also called Roseolovirus.
HHV-8
Kaposi's sarcoma-associated herpesvirus (KSHV) is the eighth human herpesvirus; its formal name according to the International Committee on Taxonomy of Viruses is HHV-8. This virus causes Kaposi's sarcoma, a cancer commonly occurring in AIDS patients, as well as primary effusion lymphoma and some types of multicentric Castleman's disease.
KSHV is a herpesvirus, and is a large double-stranded DNA virus with a protein covering that packages its nucleic acids, called the virion, which is then surrounded by an amorphous protein layer called the tegument, and finally enclosed in a lipid envelope derived in part from the cell membrane. KSHV has a genome which is approximately 165,000 nucleic acid bases in length. It is a rhadinovirus, and is remarkable since it has stolen numerous genes from host cells including genes that encode for interleuken-6, BCL-2, cyclin-D, a G protein-coupled receptor, interferon regulatory factor and many others. While no other human tumor virus possesses these same genes, many of these genes target the same cellular pathways targeted by other tumor viruses illustrating that at a basic level, all tumor viruses appear to attack the same cellular control pathways, so-called tumor suppressor pathways.
After infection, the virus enters into lymphocytes where it remains in a latent ("quiet") state. The virus exists as a naked circular piece of DNA called an episome and uses the cellular replication machinery to replicate itself. Various signals such as inflammation may provoke the virus to enter into "lytic replication". When this occurs, the viral episome starts replicating itself in the form of linear DNA molecules that are packaged into virus particles which are expelled from the cell, to infect new cells or to be transmitted to a new host. When the virus enters into lytic replication, thousands of virus particles can be made from a single cell, which usually results in cell death.
