On any given day, more than 4,000 people pass through the library at California State University-Los Angeles.
On April 11, one of them had measles. The building has only one entrance, which means that anyone who entered or exited the library within two hours of that person’s visit potentially was exposed to one of the most contagious diseases on Earth.
It’s the stuff of public health nightmares: Everyone at the library between 11 a.m. and 3 p.m. that day had to be identified, warned and possibly quarantined. Measles is so contagious that up to 90% of people close to an infected person who are not protected by a vaccine or previous case of the disease will become infected. But how could the university figure out who had been in the library during that time frame? And which of those people were vulnerable to infection?