Everyone “knows” vaccines are great, but what’s their evidence, really?

How do they know that reductions in infectious diseases is caused by vaccines and not by improved living standards?

After all, pretty much all infectious diseases were in steep decline before vaccines use became widespread.

What's to say that trend wouldn't have continued without vaccines?

In fact, it could be argued that "live virus" vaccines may well be keeping certain diseases in circulation, that may have died out years ago in their absence!