Figuring out the cause of disease was a goal for anyone practicing medicine from the beginning of the discipline. Long before germ theory was developed, the miasma theory was proposed by Hippocrates (c. 460 – c. 370 BC): that some form of "bad air" or even "bad water" arising from rotting matter caused diseases like the Black Death, cholera, and other infections. This was an important move away from the theory of supernatural causes of illness. (There was also the idea of an imbalance in the body's natural humors.) The miasma theory allowed infection to pass through a population because of the environment, not from personal contact.
To counter bad air, you would naturally want "good air." medical faculty of the University of Paris, writing in 1348 to explain the causes of the Black Death, said "The present epidemic or pest comes directly from air corrupted in its substance" and recommends incense which "hampers putrefaction of the air, and removes the stench of the air and the corruption [caused by] the stench."
Earlier, however, there were counters to the miasma theory.
The Classical Era and Middle Ages did have theories of person-to-person contact. Thucydides (c.460BCE - c.400CE) believed that the plague of Athens was being spread by personal contact. Galen (129 - c.200CE) referred to "seeds of plague" in the air. Isidore of Seville also mentioned "plague-bearing seeds." Avicenna (c.980-1037) was widely studied, and he linked the miasma theory with personal contact, believing an ill person could infect others by transmitting the "bad air" through breathing. His example was tuberculosis, and he believed that disease could also be transmitted through dirt and water, anticipating Semmelweis by 800 years.
Recent posts have mentioned Bologna as an important center for medical study, so it is not surprising that it was a professor of Bologna, Tommaso del Garbo (c. 1305–1370), who in 1345 promoted Galen's "seeds of plague" idea in his works
It took Girolamo Fracastoro in 1546, however, to publish De Contagione et Contagiosis Morbis ("On Contagion and Contagious Diseases"), three volumes on different diseases and their avoidance and treatment. He believed that there were particles that could travel through the air or by direct contact.
The idea that these "seeds of disease" were living things (what we call "germs") was not being entertained, simply because there was a long-standing theory that living matter could arise spontaneously from putrefaction. The belief that life could spring from rotting organic matter hindered understanding of bacteria already existing in the air around us. As it turns out, there were plenty of examples of Spontaneous Generation; let's talk about those tomorrow.