We hang around in undercooked and rotten food so when you dig into us, surprise! Your sick. Like E. Coli, a little sickness we give you when people don't wash vegetables or fully cook beef that you eat. Or there's Salmonella. That's what you get when you eat rotten chicken that we're hanging around on.
We can also cause sepsis in cuts and wounds. If you leave us untreated (which I think you should being a Bacteria myself) you could end up losing that limb or even dying. Yes we're that powerful. The main Bacteria that do this are Gangrene and Staphylococcus aureus.
Another way we could sneak up on you is through your untreated water. You could, of course, boil your water so we will die (But once again please don't do that). What we do is when you drink us we go into your body and cause harm to your digestive tract. Which will cause you to have a fever and diarrhea. Not to mention vomiting and stomach cramps.
Those are only three of a bunch of ways that bacteria like me can harm humans like you.
Plus, please don't do anything to kill bacteria like me. P.S. I'm the bad kind of bacteria, not those goodie two shoes bacteria that help your body, ugh.