From what I understand it is everywhere in our environment. It would be similar to testing for bacteria in your pond water. It is always there, and at some point it is going to catch you when you are down, or when conditions are favorable for infection.

Swallowing spit over extended periods of time will kill you too, at least according to George Carlin.

I for one will keep swimming in ponds over a swimming pool any day of the week. Chlorine up my nose causes me problems, and I wind up smelling phantom smoke for weeks after a swim.