The post hoc ergo propter hoc fallacy is one of the great weapons in the arsenal of denialists. The reason it works so well is it makes sense. As my readers know, my daughter is dealing with a nasty respiratory virus. One of the doctors told my wife, who is not a medical professional, that kids with this virus go on to develop asthma. My wife was not pleased to hear this. What the doc didn’t tell my wife was whether or not there is a causal relationship between the infection and asthma. It is also true that kids who get RSV end up going to school.
Asthma is a common illness. RSV is more common. Some prospective studies have followed children who have had RSV and looked at the rates of asthma, and found them to be rather high. The problem is, most children get RSV. It is very difficult to prove that a ubiquitous virus has a causal role in a common respiratory disease. The best study would follow similar children with and without RSV, but where do you find kids without RSV? How do you know that kids who don’t have clinically apparent RSV infections aren’t just resistant to respiratory disease in general? That being said, there is a fascinating literature on the effect of RSV on the lungs and the immune system. I’m actually becoming a living laboratory as we speak.
Human beings can’t help being susceptible to the causation/correlation fallacy. It’s probably hard-wired. But we can watch out for it and maintain a skeptical eye on claims of causation.