My response to a question on Reddit – Ask Science:
Let’s say you get infected with a bacterium that causes annoying, but totally non-dangerous symptoms. If you just try to “live with it,” will your immune system eventually kill it, or does killing bacteria require antibiotics in all cases?
Your body definitely kills lots of bacteria.
Your body also has tons of bacteria all the time (many doing much more good than they do harm). These bacteria also compete with each other.
So your “existing” bacteria kill off others all the time too (you have lots of different types of bacteria full time in your body – they often settle into niches and fight off any others , which is normally good as they are long term residents your body has learned to live with them).
Also like everything bacteria die off themselves – though if the conditions are right they are multiplying like crazy so that exceeds die off.
An astonishing number and variety of microbes, including as many as 400 species of bacteria, help humans digest food, mitigate disease, regulate fat storage, and even promote the formation of blood vessels.
According to estimates, phages destroy up to 40 percent of the bacteria in Earth’s oceans each day.
Staphylococcal food poisoning – an example of bacteria infection my body dealt with quickly.
People talk about genetics impact on getting cavities and impact of brushing and flossing well. Also the makeup of bacteria can help or hurt. If your mouth is home to certain bacteria tooth decay is less likely, home to others it is more likely. They tend to remain fairly steady (a certain makeup of bacteria will be consist for a person over the long term – not perfectly that way but tend that way). A UCLA microbiologist developed a mouthwash to try and ceed your mouth with good bacteria and oust the bad guys.
Related: People Have More Bacterial Cells than Human Cells – Human Gene Origins: 37% Bacterial, 35% Animal, 28% Eukaryotic