You can’t, in general, test something that already happened. Moreover, given the immune system is a complex system with some degree of randomness involved, it w

You can’t, in general, test something that already happened. Moreover, given the immune system is a complex system with some degree of randomness involved, it would probably be impossible to prove that any particular person’s allergies were caused by a too clean environment. But we can certainly study it at a population level and it can be falsified with populations. This is similar to smoking. Smoking is a good explanation for a large amount of the cases of lung disease at the population level and is falsifiable at that level. But it is impossible to say that smoking caused a particular person’s lung cancer. So my original comment of “I wonder if...” is just that. A musing, not a hypothesis. I can’t prove anything for a particular person. But maybe more people will be aware of the hygiene hypothesis and might take it into consideration.