Question

How microbes are harmful and beneficial to us

Answer 1

Some of the microbes are harmful because they birth some diseases in our body which are very harmful to us. For eg, virus causes malaria, etc.
Some microbes are useful for us for making curd etc are useful or helpful for us.

Answer 2

Here is your answer

~ Microbes become harmful :-

When that cause any disease in human body.

~ Microbes become useful :-
When they help in making curd, beverages, medicines etc.

Hope it helps you.