A passenger virus is a virus which simply uses it's host for the purpose of living, much like a parasite. Every human has thousands of passenger virus's in them, none of which cause the human to suffer any harm, symptoms, illness or disease. It is possible to create an antigen test for a particular group or sometimes a specific passenger virus, in most cases this is the only way to know if you have a specific passenger virus. Many times it is more cost effective for phamacuetical companies to produce an anti-body test to a group of virus's or passenger virus's. Some test's for instance will show both the feline(cat)HTLV-4 virus and the human HIV virus. Thus a positive test mean's your body has been exposed to either the HTLV-4 or HIV virus.

It is important to note that some peaple have an immune system that can successfully fight off a deadly viral disease and be perfectly healthy until the die of extreme old age, but they will test POSITIVE for that group of specific antigens (virus). Futhermore, if this individual was run over by a car, had thier head chopped off, or had pnuemonia and died immediately, any doctor could or would say he was "positive" for the "fill in the blank" virus and that this was the cause of death ("or a contributing factor"). To conclude that one causes the other is not always logical, doctors are not epidemeologists and will recite what they have been told.