The archaea (formerly known as archaebacteria, but presently usage of this term is discouraged), are one of the three domains of life, apart from the eubacteria and eukaryotes. They were discovered in the late 1970s by Dr. Carl Woese at the University of Illinois by genetic comparison. Archaea are prokaryotes who differ in many important respects from the eubacteria. Many archaea live in extreme environments (of course extreme from our point of view, not from theirs), like geysers, black smokers, alkaline, salty or acid waters.
The environmental conditions archaea prefer and their unusual biochemistry make them usually harmless to organisms belonging to the other two domains. No case of infection of a human with archaea has been reported so far.
See also: extremophile.