The term microorganism or microbe refers to an organism that cannot be seen with the naked eye and hence, must be viewed using an instrument such as a microscope. The study of microorganisms is referred to as microbiology. Microorganisms are almost exclusively unicellular organisms, but not all unicellular organisms are microscopic, and there are some organisms that are microscopic. Microbes can include various types of bacteria, protozoa and fungi, but not usually viruses as these are deemed to be non-living.
Microorganisms are found almost everywhere on Earth where liquid water is present. Some microorganisms called extremophiles can survive in the most hostile environments such as deserts, hot springs, the ocean floor, or at the poles.