Microorganisms can be harmful and useful yeast is a microorganisms and is used in bread but some bacteria (microorganisms) can be harmful,they can make you in or kill you,but in our imune system there are antibacteria fighting off disease and protecting microorganisms can be harmful and useful.
Microorganisms help plants fix nitrogen from the atmosphere
Microorganisms on your skin help keep you healthy by eating dead skin cells and preventing other microorganisms from causing disease.
harmful effects -
Microorganisms make you sick (viruses, bacteria), either by their very presence, or because they produce toxins that make you ill. They cause plant diseases(viruses, bacteria). They lead to cavities in your mouth. Too much of a bad microorganism can keep a good one from growing.

Microorganisms r helpful, they decompose dead and decaying matter. The useful bacteria are yeast harmful like fungi and other batericas and viruses.