BY 1917 THE MEASLES WERE WELL KNOWN, if not completely understood. No one had yet definitively pinpointed the virus that transmitted the disease, but physicians had been describing its symptoms since the 10th century, and knew that the infection largely spread among the very young.

But the outbreak in U.S. Army staging camps that year was unlike anything that came before it. More than 95,000 soldiers who were preparing to fight in Europe became infected, with recruits from rural areas—who hadn’t acquired an immunity through previous exposure—being particularly vulnerable. The disease did not normally kill healthy young adults, but now thousands of these soldiers were dying.

The epidemic struck at an opportune moment in medicine, says physician and historian David M. Morens, a scientist at the National Institute of Health’s National Institute of Allergy and Infectious Diseases, and professor at the Johns Hopkins Bloomberg School of Public Health: “The different medical specialties that had been Balkanized could now come together to solve an important problem.” Using the telegraph and the relatively new technology of the telephone—the first U.S. coast-to-coast call had been made just two years earlier—the volunteering and drafted physicians, who came from many specialties, were able to share their various insights in real time. Some data were published in the Journal of the American Medical Association fewer than seven days after being submitted.

A picture soon came into focus. Measles patients were also coming down with secondary bacterial infections, particularly Streptococcus pyogenes. The two organisms—strep bacteria and the measles virus—seemed to work in concert to create a far deadlier cocktail than either alone was capable of causing, in a process now known as co-pathogenesis.

The camps instituted aggressive infection control measures. They separated new arrivals from those already in the camp and isolated infected soldiers. Recruits gargled with disinfectant. Soldiers broke down their tents by day to improve ventilation, and some camps prohibited men from congregating in large groups in barracks and YMCAs. On sick wards, attendants wore face masks and thick rubber gloves—measures that had, until then, rarely been seen.

Victims of the disease also became the focus of pioneering treatments. Surgeons used the relatively new X-ray technology to map out regions of the lung from which they would try to clear out pus. To prevent the cardiac failure that killed many of these patients, physicians also doled out treatments that included caffeine, camphor, digitalis and epinephrine. To head off the strep, they gave out streptococcal horse antiserum—an antibody made in horses. Yet while the preventive measures made headway against the disease, these treatments were less successful. The overall response, however, laid important groundwork for battling the Spanish influenza virus that struck the very next year, infecting a third of the world’s population and killing more than 50 million people. (In fact, the “measles pneumonia” of 1917 and the “influenza pneumonia” of 1918 were at first considered one and the same: an “epidemic bacterial pneumonia.”)

Physicians had few tools to stop these viral contagions, but the military’s protocols for fighting measles were widely replicated. Health departments around the country enforced compulsory isolation and quarantine procedures, distributed face masks, closed schools and issued ordinances intended to limit public gatherings. Such efforts, says Morens, likely made a small but significant difference in rates of infection.