According to the Centers for Disease Control and Prevention, hand washing is the best way to prevent infection and illness. Hands come in contact with many bacteria and other contaminants when using the restroom, touching surfaces touched by other people, handling raw eggs or poultry, or changing a diaper. If you don't wash your hands before eating or preparing food you can ingest these bacteria, which could cause severe illnesses.
Salmonella in uncooked eggs or raw poultry can be spread to other foods with contaminated hands. Handling raw chicken, for instance, and then tearing lettuce for a salad without washing in between allows the bacteria to transfer onto the raw vegetables. The bacteria multiply on the salad at room temperature, while the chicken safely cooks, and anyone who eats the salad could get food poisoning. They will experience stomach pains, loose stools, and sometimes nausea and vomiting. In most cases this is a mild food poisoning but it can be dangerous to elderly people.
E. coli Poisoning
The E. coli bacteria spreads from the contaminated stools of an infected person. If this person uses the restroom without proper hand washing afterward, he can pass along E. coli on surfaces, to the hands of others or in food. Ingesting this bacteria causes severe diarrhea and cramping for about one week. The Partnership for Food Safety Education lists a severe complication of this infection, hemolytic uremic syndrome, that is rare but can lead to kidney failure.
Colds and Flu
Although colds and flu can spread through the air, the germs are also transferred through hand-to-hand contact. If someone with the flu or a cold sneezes into her hand, does not wash and then touches someone else's hand, the germs will spread. The person who was touched with these germs could prevent getting the virus if she washed her hands as soon as she was touched. Otherwise she risks not only coming down with the illness, but spreading it to others with whom she comes in contact.
In the health care setting, improper hand washing can have fatal consequences. Bacteria can be transferred from one patient to another through the caretaker's hands. During certain procedures bacteria may be introduced from unwashed hands into the bloodstream, causing a severe systemic infection that can lead to death.
The CDC feature "Wash Your Hands" tells of an experiment more than 150 years ago before the importance of hand washing was clearly known: A physician instructor had his students wash their hands after handling corpses and before treating mothers on the maternity ward. Previously at this clinic the death rate of mothers was five times higher than at other clinics. After the students began consistently washing their hands, "deaths on the maternity ward fell fivefold."