As a group, they are known as pathogens, agents that cause disease. It is tempting to think of viruses and bacteria as pathogens, and some of them are, but hardly all. Billions of bacteria cells live inside our bodies without causing harm. In fact, the estimates I’ve seen indicate that as few as 1 percent are likely to make you sick. And there’s a very good chance that you have cancer inside you at this moment, but it is essentially harmless. Like any good story, it can be tough to tell good from evil and indifferent.
