AIDS is caused by the HIV virus but isn't highly infectious. Unlike highly infectious viri causes illnesses, you can't get AIDS from casual contact. It's at least possible that some other illnesses like Parkinsons might be caused by some sort of a virus like a slow acting retrovirus. It could also be the result of a genetic problem. Research into both possible causes continues.

Some prominent cancer researchers believe some types of cancer might be caused by a virus of some sort. For example, the National Cancer Institute has a Viral Epidemiology Branch that investigates the relationship between viruses and cancers. Likewise, some researchers believe there may be a link between diseases like diabetes and viral infection.