Contagion can occur under terrain theory because of bad bacteria and parasites. Parasites as a source of disease is massively downplayed. There are also things that look like contagion due to sudden agricultural product dispersion or vaccinations. The main point is that what in many cases looks like disease and is treated by selling drugs under germ theory, is actually the body's immune system struggling to deal with a toxic environment because the host refuses to exercise, eat nutritional foods, kill parasites safely, and engage in periodic cleansing. It is worth considering terrain theory. My guess for things like chicken pox is that they are bacterial. In our modern world, the completely healthy person is closer to myth than not, so there are going to be times where a hybrid understanding and approach is going to make the most sense.