Microorganisms that are too small to see with the naked eye surround us and can have both beneficial and harmful effects. While some are used to produce foods and medicines, others can cause dangerous diseases. Microbiology studies these microscopic organisms, which exist virtually everywhere and can remain dormant in hosts for years before causing illness. Important early discoveries included bacteria and the role of microbes in fermentation. Though tiny, microbes can spread severe infectious diseases in plants and animals, sometimes causing large epidemics with high mortality. The invention of vaccines has helped prevent diseases caused by bacteria, viruses, fungi and other microorganisms such as cholera, tetanus, hepatitis and some cancers. Over time, continued study and discovery of new