I'm not a doctor, but consider the size difference between molecules and viruses. Some quick googling puts the size of the aids virus at 0.1 micron and the size of a water molecule at 0.018 microns if I did the conversion right.

Getting from the toilet through the anus and into the intestines is it's own challenge. There's shit, multiple sphincters and bends in the way.

Then you've got the transmissibility of aids itself, which is actually quite low for anything other than blood transfusions: http://www.aidsmap.com/Estimated-risk-per-exposure/page/1324...

Put all that together and there's no chance anyone got aids from toilet splashback.