Not upvoted cos low SP, but followed!
This reminds me of a Penn & Teller Bullshit episode (according to google, probably S02E02 Safety Hysteria), but they reached the opposite conclusion there. Glad to know my placing toilet paper on public toilet seats is more than mere germaphobia!
Pen and Tellers conclusion is correct. At least with regards to you getting infected. The bacteria are there but with out any open sores people just don't contract the diseases from the toilet seats. Now why not? That is an interesting question, because one would think that they should.