What's the meaning of this sentence

'Do you harm' means 'harm you'. In this case, it means it can make you sick.

The phrase "do you harm" can be thought of as "cause damage / injury to you". As in this example you are referring to bacteria and the human body you can think of the phrase as meaning "make you ill / sick"

A way of rephrasing the whole sentence would be:

You would be made ill by certain bacteria being in the wrong areas of your body.

I hope that helps

amgcA way of rephrasing the whole sentence would be:
You would be made ill by certain bacteria being in the wrong areas of your body.

OP question: Certain bacteria in the wrong areas of your body would do you harm.

This actually means certain organs of your body, for exmaples; respiratory track and liver, are more susceptable to vira

Thanks for your time, experts!

dimsumexpressThe original was clear and needed no rephrasing or rewriting.

The original may not have needed rephrasing / rewriting to be clear to you however the person who asked the question was unclear on the meaning of the sentence. As such, rephrasing the sentence was likely to be of help to them.