How does your immune system remember a germ for decades?

Your immune system remembers because some of the cells that fought an infection never retire. After the battle, a small squad of T cells and B cells tailored to that exact germ sticks around as long-lived memory cells. They wait quietly in your blood, sometimes for decades. If the same germ shows up again, they recognize it immediately and launch a faster, stronger attack, often before the germ can multiply to vast numbers. Antibodies made the first time also linger as an extra line of defense.

People noticed the effect long before anyone knew about cells. In 430 BC, the historian Thucydides survived the plague that swept Athens, and he wrote that the sick were cared for by those who had recovered, because no one caught it a second time, or at least not fatally. It may be the first written description of immunity.

This memory is also the whole idea behind vaccination: show the body a harmless preview of a germ, and it files the memory away. The memory isn't equally good for every disease, though. Immunity to measles or chickenpox usually lasts a lifetime, while immunity to many other diseases fades, and scientists are still working out why. Memory can even be erased: measles can wipe out existing immune memory in unvaccinated children, leaving them exposed to other infections for years.