The first polio vaccine was not until the 1950's even though polio, along with most other infectious diseases that were rampant in the early 1900s, were mostly wiped out in the US before vaccines became available, because of changes in hygiene and sanitation and diet in the first few decades of the 1900's in the US. If you watch the video, it discusses this issue extensively - the actual factors that were responsible for the vast reduction in infectious diseases, which occurred before vaccines were available in just about every single example of disease epidemic.