They then pass the bacteria on to the food that will be consumed by someone else resulting in food poisoning. E-coli is a naturally occurring bacterium that exists in the intestinal tract. While it's in there it's harmless. If consumed, it can make a person very ill and can even cause death. Making sure that hands are clean after contact with animals or fecal matter is extremely important.
Hand washing is the best way to prevent cross contamination and food poisoning.
Most people don't wash their hands long enough to kill all bacteria from their hands. A person should lather up their hands with anti-microbial soap for at least 20 seconds before rinsing. Preventing the bacteria from multiplying is very important. If food is accidentally left out at room temperature discard it. It's better to waste food than to end up sick.