The hardest part might be to eliminate the threat of infectious diseases entirely. Polio is perhaps the best illustration of this challenge: the worldwide infection rate dropped from some 400,000 cases in 1985 to fewer than 100 by the year 2000, but in 2016 there were still 37 cases in violence-beset regions of northern Nigeria, Afghanistan, and Pakistan. And, as illustrated recently by the Ebola, Zika, and COVID-19 viruses, new infection risks will arise. Vaccines remain the best way to control them.
