Generally, STDs are not passed by coming in contact with a toilet seat. Most STDs are only transmitted during sexual contact, either by skin-to-skin contact or through body fluid exchange. Crabs, or pubic lice, may be transmitted through sexual contact, sleeping in infected bedding, sharing infected clothing and possibly through sitting on an infested toilet seat. However, lice cannot survive away from the human body for longer than 24 hours. So contracting pubic lice from a toilet seat is possible, but it's unlikely.