Dont worry Annie. The situation you discribed has no risk. HIV does not live long outside the body. Plus, the chances that the person who left the blood actualy had HIV is astonomical. Plus, it dosnt sound like the blood touched you in any way. Even if it did, theres still practicaly no risk. And lastly even if there was HIV still alive in the blood (which is HIGHLY unlikely) it would not be a sufficient amount to infect someone. So relax, no way you caught HIV from a toilet seat.