Early disease fighters, such as Edward Jenner, Louis Pasteur, and William Farr, suspected if enough people were vaccinated, it could eradicate a disease. At the dawn of the 20th century, veterinarians more interested in livestock than people seized on the idea and coined the term “herd immunity.” By the 1920s, clever studies with hundreds of thousands of mice vaulted the idea into the mainstream, stirring optimism that making a fraction of a population immune could forestall a devastating outbreak.

But even the trailblazers researching herd immunity were mystified by how to deploy it in practice. This conundrum has featured in battles against many modern plagues—such as smallpox, polio, and measles. And now it is part of the debate as