Most all of us carry opportunistic bacteria in our upper respiratory and upper/lower digestive tract. These bacteria can quickly become pathogens given the right circumstances - i.e. illness. Ever notice how many people develop secondary infections after a course of antibiotics? Most antibiotics given these days are broad spectrum, and end up killing most all the bacteria in our bodies, good and bad. Commensal bacteria are normal flora and actually help us prevent infection by competing for the same space and resources that the potentially bad bacteria are looking for, which is why it's important to maintain a healthy immune system to preserve that delicate balance.

Washing your hands is VERY important, but remember that it only rids your hands of transient bacteria/viruses, and does little in the way of killing bugs that are normally present. Germicidal soaps/hand cleansers - NOT antibacterial soaps mind you - actually kill the bugs. Avagard makes a nice product line of germicidal hand cleansers used by most hospitals.