Beer doesn't have a high enough alcohol content to kill most infections, you need about 50% alcohol (100proof) for an effective disinfectant. The fermentation process will kill most infections because the yeast you intentionally put in out competes other things that find their way into your brew. Most infections that commonly occur (wild yeast, mold, and bacterial infections are possible) are not dangerous, a few are beneficial like the bacteria that works with the yeast to make kombucha, or lactobacillus for sours and most fermented foods, or penicillium mold. If your brew becomes infected the most likely result is that it will taste and smell bad and maybe cause some gastrointestinal upset but not be particularly dangerous. For homemade alcohol distilling is where it really gets dangerous. Every fermentation will create some methanol but a healthy fermentation will produce very little and it will mostly get cleaned up during aging. A bad fermentation, either caused by infection or bad yeast, will produce more and in beer the levels will cause an unpleasant taste and smell but not hit toxic levels. When you distill a beer with higher levels, it gets retained and concentrated along with the ethanol and can cause serious illness.