Since low doses of bacteria can cause infection, why aren't we always sick?

Because we have an immune system.

Low numbers of bacteria that we are exposed to, and that get into our bodies, cannot cause disease unless they multiply like crazy. And they can't do that easily if our immune system spots them when they first enter the body and destroys them.

If our immune system is so good, then why do we ever get sick from bacteria? Our immune system isn't perfect.

For my entire life, like everyone else, I've been exposed to bacteria in the world around me. Like most everyone, I've occasionally gotten a minor bacterial infection, like a sinus infection or skin infection.

But I had never gotten a serious bacterial infection until about 10 years ago. I was suddenly hit by symptoms that made me pretty sure I had developed a bacterial infection that had gotten into my bloodstream. It's called bacterial sepsis, and it can kill you. Fortunately, I recognized what was happening, got treatment very early, spent a day in the hospital, and was back to my old self in a week.

Why did my immune system fail me? I don't know for sure, but here's my theory. Two days before the bacterial sepsis hit me, I had flown from Australia to Boston. I had not gotten a good sleep in nearly 24 hours, and when I came home I was in a time zone that was the opposite of what my body expected.

I think my immune system was not strong because of my physical stress and what flying half way round the world did to my body.