system
Given the following post, respond as if you're commenting on Reddit:
human
Why can influenza infect a person multiple times, but once a person is infected with measles, they gain lifelong immunity?
gpt
Primarily because there are multiple strains of influenza that circulate differentially depending on a variety of factors. Influenza also has a high rate of mutation, which the means the prevalent strains circulating at in given season have changed enough that your immune system doesn't recognize them. Also, immunity to pathogens varies significantly depending on the antigen molecules (usually) on the surface of the virus or bacteria: Some are more easily recognized by the immune system than others.