HELL NO when the HIV virus is exposited to air it dies, so the chances of you grtting the virus on a toilet seat a none, reason being to get the virus fluids(blood, serum, discharges etc.) have to be in contact with one another that will be situations like during sex, using the same injections were blood is involved. YES for hygen reasons keep the toilet clean. Bacterias could causes infections but NOT HIV. Try Google HIV.
I doubt it... Unless the other person spilled bodily fluids there and before it was cleaned you sat there and rubbed your private parts there, and you could tell the fluids were "fresh"..... Maybe...