When you get a cold, or the flu, or other viral illnesses, your immune system adapts to keep you from getting that particular strain of illness again. So, too, with vaccines, which essentially train your body to fight off infection from the virus or bacterium they’re designed to protect you from.
Except your immune system doesn’t always cooperate. Some vaccines need booster shots over time, and some people - especially the elderly - are susceptible to diseases they would not have caught at another time in their life. So what’s going on here?