Each of us hosts a universe of bacteria in our bodies.
Many bacteria live naturally in our bodies without causing disease.
People undergoing chemotherapy, for example, who are immune compromised, people with HIV infections who are immune compromised, can also be vulnerable to some of these organisms that are often not pathogenic to people who are healthy. Dr. Cummings is trying to find the genetic differences that allow some bacteria to cause disease, while others can't. He says some disease-causing bacteria evolved from other, more ancient, harmful bacteria. But the story seems to be different for the bacteria that cause tuberculosis. There's genetic evidence, it's been estimated, that tuberculosis may have only been in humans for the last 15,000 years or so.