Like Typhoid Mary, this patient turned out to be unwittingly contagious. Ultimately, she transmitted the measles to four other people, according to a recent report in Clinical Infectious Diseases that tracked symptoms in the 88 people with whom “Measles Mary” interacted while she was sick. Surprisingly, two of the secondary patients had been fully vaccinated. And although the other two had no record of receiving the vaccine, they both showed signs of previous measles exposure that should have conferred immunity.

So how did this happen? The woman's immune system, the Daily Beast reports, "responded as though it had never seen the virus before." The measles vaccine works 95 percent of the time, which is why people are given two shots—just in case they fall into that five percent category. It turns out that the woman in question was a "statistical anomaly," the Daily Beast writes. But what about the people she infected who had also received the vaccine? Why did they, too, contract the disease? The Beast:

The report suggests that vaccinated people may lose their immunity as they age. In the past, measles in the community effectively provided a booster for vaccinated people. Now that the virus is mostly gone, their immune systems may be “forgetting” the disease, [epidemiologist George] Rutherford said. 

Experts told the Daily Beast that, rather than freaking out and giving everyone a measles booster shot, a better course of action would be to make sure everyone is vaccinated in the first place. This case, experts remind, is an extreme anomaly. According to the CDC, 80 percent of measles cases in the U.S. are contracted by people who have not been vaccinated, and many of those cases are picked up when traveling abroad to places with higher rates of measles. When the unvaccinated person brings measles back to the U.S., problems ensue, especially if she or he comes into contact with other unvaccinated people. As the Daily Beast notes, "Measles is very contagious, and in unvaccinated settings, one patient can transmit it to 9 of 10 others with whom the patient has contact."