Gregory Cochran writes in 2012 Edge question series on "What is your favorite deep, elegant, or beautiful explanation?"
Germs Cause Disease
The germ theory of disease has been very successful, particularly if you care about practical payoffs, like staying alive. It explains how disease can rapidly spread to large numbers of people (exponential growth), why there are so many different diseases (distinct pathogen species), and why some kind of contact (sometimes indirect) is required for disease transmission.
In modern language, most disease syndromes turn out to be caused by tiny self-replicating machines whose genetic interests are not closely aligned with ours.
In fact, germ theory has been so successful that it almost seems uninteresting. ...
A huge amount of money has been devoted to searching for the genetic causes of slow-acting diseases because with the development of genome sequencing we have a very handy lamppost to search for our keys under. Looking for germs that might cause slow diseases has not been a priority because nobody has much of a plan for how to do it.
This isn't a terribly bad strategy. It's like when I'm playing golf and I slice my teeshot toward a tangle of head-high thornbushes. I might go look for my ball on the next fairway to the right: maybe the ball happened to bounce through all the thorns an on to the short grass of the wrong fairway. That probably didn't happen, but if it did, well, I can find my ball a lot more easily than if it's in the thorn bushes. But, when my ball doesn't turn up sitting pretty on the next fairway over, I try not to be disappointed.
Similarly, with big diseases, the balls aren't sitting up on the short grass where it would be easy to find them. The 21st Century hasn't seen a lot of cases of common diseases being caused by common gene variants, just as Cochran predicted back in the 20th Century. But few are thinking about how to wade into the thorn bushes to look for them.
It is still worth studying—not just to fight the next plague, but also because it has been a major factor in human history and human evolution. You can't really understand Cortez without smallpox or Keats without tuberculosis. The past is another country—don't drink the water.
It may well explain patterns that we aren't even supposed to see, let alone understand. For example, human intelligence was, until very recently, ineffective at addressing problems causing by microparasites, as William McNeill pointed out in Plagues and Peoples. Those invisible enemies played a major role in determining human biological fitness—more so in some places than others. Consider the implications.