To anyone trained in medicine or biology, this question seems laughably naive. One’s impulse is to brush it off with a quick and curt answer.
But it is a very good and important question because it distinguishes modern from ancient disease concepts. The notion that specific agents cause specific diseases—something that now seems blindingly obvious— was only accepted by the medical establishment about 150 years ago.
Before that, mainstream medical thought, both East and West, conceived of disease as an imbalance of forces within the human body. Those forces might be called humors in the West, or qi in the East, but the basic concept was similar, and so was the theory behind medical intervention.
From Yin Yang Application in Pathological Changes of Chinese Medicine
From The Four Humors of Hippocratic Medicine
If the patient was warm and dry, medicines were given that nudged the body toward the cool and the moist. If the flow of energy was blocked, the patient was treated with an eye toward restoring the proper flow. Either way, the focus was on restoring the patient, not on battling disease-causing agents.
Disease originated in the patient; it was not the result of extrinsic forces, except to the extent that these forces created imbalance. The notion that outside agents cause disease was considered a throwback to demonic possession, an ignorant superstition that educated physicians had battled since the time of Hippocrates.
There always were dissenters of course. The Roman nobleman Marcus Terentius Varro, writing in the first century BCE thought that “…there are bred certain minute creatures which cannot be seen by the eyes, but which float in the air and enter the body through the mouth and nose and cause serious diseases.” That’s a concise summation of Germ Theory.
But with no actual evidence for this theory, there was no reason for anyone to take it seriously. And it was not at all clear how these minute creatures, even if they did exist, worked to cause disease. Even after microscopes made it possible to detect them, there was no correlation of their presence with disease. After all, you could detect these “animalicules” in just about any sample examined: in food, in water, in healthy tissue as well as diseased. If micro-organisms cause disease, then we should all be sick, all of the time.
Symptoms like fever—which results not only from infection, but conditions such rheumatoid arthritis or hyperthyroidism—could not be seen as manifestations of specific disease because they were the disease. Diseases could not be classified in any coherent scheme because they were just collections of symptoms, and symptoms vary between patients and over the course of a disease. Ancient and medieval physicians were extremely acute observers of their patients, but all that observation tended to obscure rather than enlighten. Data without theory is just a bunch of lifeless facts.
Not that there were not plenty of theories to explain diseases by the mid-19th century, infectious diseases especially. The micro-organisms found in diseased tissue were thought by many to be degraded human cells, or to be harmless parasites feeding on already-killed tissue. Others championed a theory of infection in which phlogiston spread from one patient to another. Or, incorporating the new discoveries in physics, that disordered molecular motions caused the breakdown of human tissue and could spread from one tissue to another—not so different from the prion theory of encephalopathy today. All of these theories were championed by very smart men (and women) and had a wealth of observational and experimental data to support them.
The critical breakthrough, as is so often the case, was not better theories but better tools. Many apparently convincing demonstrations of spontaneous generation or germ-free transmission of disease were simply artifacts of poor technique. The key tools, the reasons why we recognize and revere Pasteur and Koch today, were techniques of pure culture and of differential staining of bacteria. Boring drudge work, as any microbiologist can tell you. But absolutely crucial.
Only when it was possible to distinguish and recognize different bacteria and to grow them in pure culture did it become clear that their different shapes were truly characteristic and not a response to environmental conditions (although sometimes it is indeed just that).
And when those pure cultures of Bacillus anthracis or Streptococcus pyogenes were introduced into sheep or mice or rabbits and caused the same disease, time after time—only then was the causal link between specific entities and disease first established. Diseases could now be understood as discrete and distinguishable phenomena with different discrete and distinguishable causes.
A new theory of medicine followed directly: eliminate or neutralize the disease cause to cure the disease. That paradigm is what distinguishes modern from ancient medicine. It is the paradigm of every modern therapeutic intervention: antibiotics (obviously), statins, diuretics, insulin, cytotoxic and targeted cancer therapies; the whole list. Germ theory allowed us to connect diseases with causes, and to target those causes for cures.
Like all paradigm shifts, it went from laughable to debatable to obvious in just a few years. Of course strep throat is caused only by Streptococcus bacteria and of course those bacteria cannot cause other diseases. But this is a very modern and very recent understanding. We should recognize it and cherish it for what it is: an idea that has done more to reduce human suffering than any other.
For a thorough history of the evolution of medical thought, Roy Porter’s The Greatest Benefit to Mankind is an unparalleled resource. It spends too little time on Eastern traditions, and is more encyclopedic than analytic. But I guarantee that you will learn a great deal from it.