Like most other diseases and illnesses, cases and fatalities DROPPED as humanity invoked superior hygiene and sanitation.
Disease and illness was never widespread, generally speaking, unless there was a mix of races and cultures.
As time passed and The Agricultural Revolution came to be, we built villages, towns, and cities.
These cities became PLAGUED with disgust — people would literally throw urine and feces out of their windows onto the street next to them.
Local water sources were also heavily contaminated, leading to Cholera and Dysentery.
As we became focused on proper filters, cleanliness, and Germ Theory, we SIGNIFICANTLY reduced disease, way before modern vaccines were an invention; Measles, Mumps, and Rubella is no different.