Stanford University Stanley Falkow
Ninety percent of the cells humans carry are microbes. Only a few of the bacteria we encounter are pathogenic and can cause disease. Pathogens possess the inherent ability to cross anatomic barriers or breach other host defenses that limit the microbes that make up our normal flora. A significant part of human evolution has gone into developing ways to thwart microbial intrusion. In turn, microbes have come up with clever ways to avoid and circumvent host defenses but human — microbe interactions is still a "Work in Progress." When we study pathogens we learn as much about ourselves as we do about them.