It’s not always easy figuring out how a pathogen causes disease. There is an example in mice for which the solution was very difficult, so difficult that we would probably have failed to discover the cause of a similarly obscure infectious disease in humans.
Mycoplasma pulmonis causes a chronic obstructive lung disease in mice, but it wasn’t easy to show this. The disease was first described in 1915, and by 1940, people began to suspect Mycoplasma pulmonis might be the cause. But then again, maybe not. It was often found in mice that seemed healthy. Pure cultures of this organism did not consistently produce lung disease – which means that it didn’t satisfy Koch’s postulates, in particular postulate 1 (The microorganism must be found in abundance in all organisms suffering from the disease, but should not be found in healthy organisms.) and postulate 3 (The cultured microorganism should cause disease when introduced into a healthy organism.).
Well, those postulates are not logic itself, but rather a useful heuristic. Koch knew that, even if lots of other people don’t.
This respiratory disease of mice is long-lasting, but slow to begin. It can take half a lifetime – a mouse lifetime, that is – and that made finding the cause harder. It required patience, which means I certainly couldn’t have done it.
Here’s how they solved it. You can raise germ-free mice. In the early 1970s, researchers injected various candidate pathogens into different groups of germ-free mice and waited to see which, if any, developed this chronic lung disease. It was Mycoplasma pulmonis , all right, but it had taken 60 years to find out.
It turned out that susceptibility differed between different mouse strains – genetic susceptibility was important. Co-infection with other pathogens affected the course of the disease. Microenvironmental details mattered – mainly ammonia in cages where the bedding wasn’t changed often enough. But it didn’t happen without that mycoplasma, which was a key causal link, something every engineer understands but many MDs don’t.
If there was a similarly obscure infectious disease of humans, say one that involved a fairly common bug found in both the just and the unjust, one that took decades for symptoms to manifest – would we have solved it? Probably not.
Cooties are everywhere.