HIV is a retro virus that has a very short lifespan especially when exposed to air.
For someone to acquire HIV from a toilet seat there would have to be some extremely FRESH biological matter present (semen) and the person who sits on this deposit of matter would have to have an open wound that makes direct contact with the semen.
I say semen because the concentration of the HIV disease in other bio waste is not high enough to be viable.
To sum it up......it is close to impossible to get AIDS (HIV) from a toilet seat.
Supporting answer from WikiAnswers contributor:
Someone leaving you...Or using a toilet. Or throwing your trash out.
"Stay" by Lisa Loeb
You can't get PID from a toilet used by someone who has it.
Because someone is using water somewhere else. For instance, if you are having a shower and someone flushes the toilet, the water going to the shower is decreased because some is going to the toilet now.
In Texas, there is no specific law that prohibits using toilet paper to decorate a house. However, using toilet paper to intentionally vandalize or damage someone's property could be considered criminal mischief under Texas law. It's always best to obtain permission before using any materials to decorate someone else's property.
The chance is very small. Once herpes hits air the virus will die almost instantly. Herpes isn't likely to rub on the surface of a towel, toilet, clothing or other items. If the herpes virus happened to rub on the surface of an item the virus would die before a person came in contact with the virus. Some studies report herpes could spread by sharing a drink but the chance is almost non existent. You can get genital herpes through genital-genital contact or genital-oral contact with someone who has herpes infection. The virus is most easily spread through contact with open sores. But you also can get the virus from skin that does not appear to have a sore. You can become infected with the herpes virus without having intercourse.
Muli kaea (Samoan word) can be referred to a person who has not used a toilet paper after using the toilet, same thing you can call a someone who poo in their pants.
herpes
You can get herpes from someone who has the infection. It can affect drug users and abstainers.
Herpes can only be passed through direct skin-to-skin contact with the infected area such as kissing, intercourse, genital-to-genital rubbing, vaginal intercourse.Herpes (both oral & genital) can be spread even when there are no symptoms or sores. This is called asymptomatic shedding.In case you have herpes, I recommend you to watch the #1 way to get rid of herpes Permanently - many.link/NewHerpesCure
Someone who is having unprotected sex would be at risk for contracting herpes. Oral herpes can also be transmitted by contact with the infected area.
Using the toilet is better than the alternative.