When I first learned about balanced pathogenicity back in the 1980s, it made me feel good about the world. This is the process in which germs evolve into milder forms over time. Natural selection favors the milder strains of germs because they can spread more readily. Any germ that kills its host is at a disadvantage. There are many examples in which diseases used to be very virulent, but today they are milder even without vaccination or medication. Examples include smallpox, which in Europe and North America evolved into a mild disease; leprosy, which today is a slow skin disease but used to kill people quickly. There are even diseases such as the “sweating sickness” that had major outbreaks in Europe in past centuries but appears to have evolved itself out of existence (it persists only in very mild forms): there are no diseases today that have exactly those symptoms. Balanced pathogenicity was part of the balance of nature in a blessed world.
Or so I thought. That’s what I wanted to believe.
Then I started learning about the exceptions. Waterborne diseases such as cholera do not evolve into milder forms. Insect-borne diseases may evolve into even worse forms. So I had to change what I taught and wrote: balanced pathogenicity applies to diseases that spread to a new host by close proximity to the victim. My main example was ebola, which, I thought, will evolve into a milder form since the worst forms of it keep healthy people from coming in close proximity to the victims.
But it turns out that even ebola can evolve into a worse form, as explained in this article by Carl Zimmer. I suppose that this evolution of worse forms of ebola is a temporary reversal of the overall trend of balanced pathogenicity. But I am now having to make so many “exceptions to the rule” that I am beginning to wonder how much of a pattern balanced pathogenicity really is.
My original feeling about balanced pathogenicity came about because I wanted to believe that there was a fundamental goodness to the world. Bad things happen, but within them is the seed of a better world. This was partly because I got my optimism from the same source that I got my original information: Rene Dubos. I learned about balanced pathogenicity by reading his Man Adapting and Celebrations of Life. He was a scientist and informal philosopher in the same mold as Lewis Thomas. A great thinker. But his “gospel” was that evolution ultimately produces a better world. I wanted very badly to believe that evolution was a good process that God incorporated into a good world. But the world in which evolution works for the Greater Good is more of a Shangri-La than a real world.
Balanced pathogenicity happens, except when it doesn’t. Evolution makes the world better, except when it doesn’t. Creationists look for scientific reasons to believe God is good. Theistic evolutionists look for scientific reasons to believe that evolution produces goodness that God intended. In this particular sense, I am not sure that theistic evolution is much of an improvement over creationism.