The most dramatic example came two years ago when a group of researchers reported finding a mouse-related virus called XMRV, a pathogen in the same family as HIV, which causes AIDS.

An estimated one in two of working prostitutes is living with the virus and the lack of medication led to a quarter of a million people dying of Aids-related illnesses there last year.

Equine infectious anemia virus (EIAV) is a lentivirus. It related in sequence to the human immunodeficiency virus (HIV) and reacted with acquired immunodeficiency syndrome (AIDS) patient sera.