They can live in temperatures well below freezing to above boiling. Others can live in very acid or very alkaline environments and in the presence of chemicals or radiation that may be lethal to other organisms. Fossilized microbes have been dated to be up to 3.5 billion years old.

Viruses, which are more numerous than bacteria, have none of the properties of life. Although most people associate microorganisms with disease, the majority are actually beneficial and in some cases are used to produce economically important materials, or as vectors for the treatment of disease.