How long does immunity last after an infection or a vaccine?

It varies enormously by disease, which is why there is no single answer. Measles infection or a complete vaccine course gives lifelong protection for almost everyone. Tetanus needs a booster roughly every ten years. Influenza immunity fades within a year and the virus changes anyway, hence annual shots. COVID immunity against severe illness holds up considerably better and longer than immunity against catching it at all. Whooping cough immunity wanes over years, which is why adults transmit it to infants. The practical takeaway: 'I had it once' is not a reliable plan for most infections.