Miasma theory was a prevalent scientific theory that believed that most contagious diseases had an environmental cause. It was well supported in science until the work of Robert Koch in the late 19th century largely confirmed the germ theory of disease that was first proposed by Louis Pasteur. It lives on in modern environmental medicine and in the name of such diseases as malaria which were thought to have an environmental cause. Koch's work definitively showed that certain micro-organisms caused certain diseases, and miasma theory fell out of favor.