How vaccines work: how a shot trains your immune system

Some old people on the Faroe Islands had measles in 1781, when they were young. When it came back 65 years later and made most of the islands sick, they stayed well. A vaccine gives your body that memory without the illness. Follow one measles shot into your arm and meet antigens, antibodies, B cells and memory cells. Then see what else a shot can hold, from a killed virus to mRNA, why your arm gets sore, why the flu shot changes every year, and how herd immunity protects a baby.