many cells are quite content to stay in their region or organ, their area of the Festival of Life. Pathogens break through the barriers. Bacteria, for instance, can have little tails, called flagella, little motors that give them bursts of acceleration. A salmonella bacterium, for instance, swallowed with food, might use this propulsive tail to burst through the lining of the gut and into the body. It is built to invade.
