We're exposed to microorganisms all the time. We have them in our mouth. We breathe them in. And for the most part, people don't get pneumonia from these causes. When you do get pneumonia, it's when your host defenses breakdown and the microbes enter into your lungs. And for whatever reason, meaning that the microbe is particularly virulent, meaning highly infectious or you've inhaled a large inoculum, meaning a large quantity of the infection and that your host defense just can't fight against it effectively. And then you develop pneumonia from this.