[Mariana (OP)]

Paul wants to know:

How does the body know which bacteria friendly and unfriendly?

And, is there a certain amount of bad bacteria or viruses that are good for the body to experience to build up a tolerance to (e.g, if someone sneezes near you, what drives you to either get the illness or not?

Can a little bit of exposure be a good thing?

Do you know the answer?

[evan_au]

How does the body know which bacteria friendly and unfriendly?

The unfriendly ones cause cell damage, which promotes inflammation and attracts the white blood cells.
- The white blood cells latch onto anything suspicious, and recruit more white blood cells to come and kill whatever the first lot found.
- If all goes well, all these white blood cell lines will have been trained in the thymus so they don't attack your own cells (which can trigger an auto-immune disease). 

See: https://en.wikipedia.org/wiki/Adaptive_immune_system

Can a little bit of exposure be a good thing?

The "Hygiene Hypothesis" suggests that our over-sanitised lifestyles may produce a "bored" immune system; with no real enemies to fight, it sometimes picks on your own body or innocent chemicals in the environment, triggering things like Type 1 diabetes, asthma, Crohn's disease, peanut and other allergies.
See: https://en.wikipedia.org/wiki/Hygiene_hypothesis

Some people with auto-immune diseases have self-medicated by infecting themselves with parasitic worms. These worms excrete substances to damp down the immune system.
https://en.wikipedia.org/wiki/Helminthic_therapy