Henry R.S. adds (14.9.03):

You can also catch crabs and genital warts.

Genital warts are the easiest nasty thing to catch from toilet seats. Though
the most common thing to catch from a toilet is flu or a cold. Every time
someone flushes a toilet and doesn't put down the lid a plume of germs are
airborne on moisture making it easy for them to be inhaled and passed on.

Noel writes (6.03):

Lots of bad stuff like Salmonella and E Coli are
carried in the intestine and can cause food poisoning
if ingested (bizarrely enough). I've just read Fast
Food Nation and it has a very good section on this.
Fun facts (from memory):

- American hamburger meat is something like 10% shit

- The average American toilet is cleaner than the
kitchen sink