Human herpesvirus 8 (HHV-8)

Human herpesvirus 8, or HHV-8, is a virus whose genetic material is double-stranded DNA, the molecule that holds its instructions. It passes from person to person mainly through saliva, and the infection usually goes unnoticed: the person never even realizes it.

What makes it unusual is that it can stay in the body for life. It settles into B lymphocytes, a type of immune cell, and into the vascular endothelium, the thin layer that lines the inside of blood vessels. There it stays latent, meaning quiet and inactive. It's like a tenant who moves into a back room and hardly makes a sound: it may never leave, and every so often it starts multiplying again. That happens mostly with immunodeficiency, meaning when the body's defenses are low. The virus also makes proteins that help it hide from the immune system, and it has the potential to promote cancer.

Then there are the diseases it causes. One is Kaposi sarcoma, a cancer of the blood vessels. Others are diseases in which B lymphocytes multiply too much. HHV-8-related cancers are much more likely in people living with HIV, the virus that weakens the immune system, but they also show up in people who don't have it.