Shigella bacteria passes very easily among people, especially when you forget to wash your hands (or if you're among the 95 percent of people who don't correctly wash their hands). These bacterium causes shigellosis, but you're probably more likely to recognize one of its trademark infections: dysentery. Shigellosis causes infectious, severe diarrhea, abdominal cramping and other gastrointestinal distress that may last for about a week (although some types, such as dysentery, are known to be nasty enough to cause epidemics). And while the bacteria are most infectious during the diarrhea phase of the condition, they remains willing and able to infect you for weeks after their host is feeling better.
Shigella infections, similar to E. coli, happen when an infected person's feces contaminates a surface -- and, yes, those surfaces include toilets, toilet handles and toilet seats. You can also become infected if you consume contaminated food or water handled by an infected person who hasn't practiced good hygiene. Not to scare you, but about 14,000 cases are reported in the U.S. every year [source: CDC].
The best way to keep a toilet and its seat free of shigella bacteria is to clean it with bleach. Otherwise, keep the bacteria at bay by washing your hands, and if you just don't trust that toilet seat, wipe it down with a disinfectant or antibacterial wipe before you sit.