Germ Theory Denialists
The idea that many diseases are caused and spread by microorganisms has been fairly well understood since the mid 19th century. Even before that, it was well known that you could get a disease simply by breathing air around a sick person (though the belief was that it was the miasma or bad air itself that caused it). The ancient Greeks theorised that disease could be spread by various 'seeds' that could be breathed or eaten on food.
Although Louis Pasteur's model of infectious diseases is now well accepted and understood, there were some scientists who thought he was wrong. Pasteur's contemporary Antoine Béchamp proposed the alternative Microzymas Theory where the micro-organisms are part of the host's body and unhealthy pursuits or bad diet causes them to change or break-out. This change then manifests as disease. Correcting the environment or diet stops these changes and thus cures the disease.
Despite germ theory being the predominant scientific consensus around the world, amply demonstrated countless times, there are still germ theory deniers who do not accept the preponderance of evidence. Often it comes down to the fact that alt-med practitioners may have their own theories about the causes of disease, usually based on Béchamp's ideas of something in the patient herself causing the issue, which can then be 'corrected'. For chiropractors it is spinal curvature, for others it may be misalignment of chi or meridian lines. Some say it is all down to diet and promote eating all fruit/meat/paleo or whatever fad diet as a cure-all. Some go so far as to deny the need for hand-washing or good hygiene. Of course the actions a patient takes can hugely influence their susceptibility to germs, but this is not what denialists mean.