The basic premise is that a whole community (or "herd") can be protected from a disease so long as a high enough proportion of the population is vaccinated. This prevents the bacteria or virus causing the illness from spreading. For measles, which is highly transmissible, 90 to 95 percent of people should be vaccinated to achieve herd immunity but there are some parts of Europe where as many as 30 percent of the population is unvaccinated.
Similarly, there are "hot spots" in the US where well under the required 90-95 percent of children are being vaccinated. In Camas County, Idaho, for example, more than one in four kindergarteners are unvaccinated.
"There is a terrific vulnerability in states like Texas and up in the Pacific Northwest,” Hotez added. “People forget that before kids were getting vaccinated we had between 400 and 700 deaths from measles annually in the US.”
While there have been measle outbreaks in the US (New York in 2013 and Minnesota in 2017), they have not been on the scale of the one currently sweeping Europe. But that doesn't mean there isn't a much larger outbreak waiting to happen.
"This is a real setup for disaster since measles is insanely contagious," Dr Albert W. Wu, a professor of health policy and management at the Johns Hopkins Bloomberg School of Public Health, told NBC.
"This is an accident waiting to happen."