So you need a bit of knowledge about what keeps you from getting sick, but actually preventing the sickness is very, very easy. In fermentation, there are several ways of doing this, but salinity and acidity are the most important factors, and those are built into your fermentation recipes. Kahm yeast can be scooped off the top when you see it form, and while molds can (but don't always) ruin a ferment, they're uncommon if you're maintaining proper salinity, using fermentation weights, and starting with very clean equipment.

You don't leave your ferments uncovered, and while you can do fermentation in ways that you'd need to worry about pests, cheap fermentation equipment that will make the question irrelevant is easily available.