Question: who discovered that infectious disease was spread by bacteria

title: Germ theory of disease
text: The germ theory was proposed by Girolamo Fracastoro in 1546 , and expanded upon by Marcus von Plenciz in 1762 . Such views were held in disdain , however , and Galen 's miasma theory remained dominant among scientists and doctors . The nature of this doctrine prevented them from understanding how diseases actually progressed , with predictable consequences . By the early nineteenth century , smallpox vaccination was commonplace in Europe , though doctors were unaware of how it worked or how to extend the principle to other diseases . Similar treatments had been prevalent in India from just before AD 1000 . A transitional period began in the late 1850s with the work of Louis Pasteur . This work was later extended by Robert Koch in the 1880s . By the end of the 1880s the miasma theory was struggling to compete with the germ theory of disease . Eventually , a `` golden era '' of bacteriology ensued , during which the theory quickly led to the identification of the actual organisms that cause many diseases . Viruses were discovered in the 1890s . 

Answer: answer: The germ theory was proposed by Girolamo Fracastoro in 1546, and expanded upon by Marcus von Plenciz in 1762. Such views were held in disdain and Galen's miasma theory remained dominant among scientists and doctors. A transitional period began in the late 1850s with the work of Louis Pasteur.