To see why, look to hormones and genetics, said Sabra Klein, professor of microbiology and immunology at Johns Hopkins University. Sex hormones appear to play a role determining how well human bodies can fight off disease. In general, estrogens amp up the immune system, while androgens (like testosterone) and progesterone suppress it. Hormones have to interact with cells to do their jobs, Klein said, so cells have a “lock” and hormones have the “key” to get in. And, turns out, every immune cell in your body has these kinds of lock-and-key receptors.

Sex chromosomes also play a role. The X chromosome, for example, has 60 genes associated with immune function. Most biological males are born with one X chromosome, but those immune-boosting genes tend to be expressed more frequently in women, who generally have two X chromosomes, Klein said.

But if you’re tempted to think those sex differences boil down to “men are more likely to die from infectious disease,” Klein cautioned that the generalization is not true across the board. Sex differentiation in influenza has been pretty well studied, she told me, and, in that case, women’s more aggressive immune systems don’t give us an advantage.