A microorganism is an organism or biological entity that is too small to be seen clearly with the unaided eye.
Microorganisms include bacteria, fungi, protozoa, and many other microscopic forms of life. Some microorganisms live harmlessly on or in humans and can contribute to normal body functions, while others can cause disease.
Viruses are commonly discussed alongside microorganisms because they are microscopic infectious agents, although whether viruses are classified as organisms is a matter of biological definition.
QUICK FACT
Not all microorganisms are harmful. The human body normally hosts vast communities of microorganisms, many of which are harmless or beneficial.
COMMONLY CONFUSED WITH
Infectious agent: A microorganism may be an infectious agent, but not every microorganism causes infection.
SEE ALSO
