Answer:
Microorganisms that are not visible are so called because their measurements are in micrometers and even nanometers.
We usually talk about viruses, bacteria, fungi and even parasites.
Step-by-step explanation:
Microorganisms are mostly pathogens for humans, but others make up the commensal biota of the same living being.
Not all microorganisms need the same means to live and develop as large organisms such as those that are simple to the human eye.
Some microorganisms are seen by optical microooscopy and others smaller by electronically.