A Answers (1)
Anthony Komaroff, MD, Internal Medicine, answeredEach time a flu virus reproduces, its genes mutate. This happens again and again, until the surface of the virus at large in the community looks so different from the original virus that it can infect the same population all over again. By significantly changing its appearance, the new strain of virus is able to sneak past the immune system's surveillance mechanisms. This gradual process is called antigenic drift.