Probably the biggest thing you ever have to worry about from a toilet seat is the transmission of diseases like cholera and hepatitis that can be passed through direct contact with infected feces. If, however, the feces goes directly into the bowl and is then flushed away, then we have nothing to worry about. It is only when the infected person is falling all over the bowl in an attempt to avoid catching some phantom unknown pathogen that disease is truly likely to be spread. Crabs, also known as pubic lice, can theoretically be picked up from a toilet seat, but again the chances of that are incredibly slim. You would be more likely to pick up pubic lice from using a towel an infected person had just used.
Lice tend to stay embedded in the warm, dark areas where they thrive.
Bodily contact spreads lice easily because the parasite transfers from one friendly environment to another. In other words, we have nothing to fear but fear itself. Making contact with urine, while extremely unpleasant, especially when it is not your own, does not expose you to disease of any kind. In fact, urine is used in some parts of the world as a disinfectant and is thought to have antibiotic properties. Some people believe in drinking their urine and report definite health benefits.