The latter are called “live, attenuated vaccines,” and over the past century, scientists have noticed something peculiar about these vaccines: They seem to offer some protection, not just from the targeted disease, but also against many different diseases, including respiratory infections.

“There’s plenty of evidence for it,” Gallo says.”The weakness is we don’t really know the longevity [of the protection]. It will probably work only for months, but we can’t say for sure.”

Take for instance, the vaccine for tuberculosis. It’s called bacille Calmette-Guerin, or BCG, and it contains a live, but weakened, strain of TB from cows.

When doctors in Sweden first started using BCG back in the 1920s, they noticed the vaccine not only reduced a child’s risk from dying of TB, but that children who got it had a mortality rate from all causes that was almost three times lower than unvaccinated children. Since the 1970s, scientists in West Africa have documented a similar pattern with both the BCG vaccine but also the live measles vaccine. In other words, they were doing something to boost the immune system’s response to many kinds of pathogens.