Much onus for this regression so far has been laid on the global anti-vaccination movement. But experts blame much more sweeping reasons, primarily a tectonic change in how humans live now as opposed to three, four and five decades ago.

At the top of the list: we are living closer together in ever-swelling cities, trading and traveling much more, creating climate change, migrating in big numbers — and failing to keep vaccination levels high enough for “herd immunity.”

“What changed is that society changed,” Jeremy Farrar, an expert on infectious disease and director of the U.K.-based Wellcome Trust, tells Axios.