Question

explain how natural selection led to flu illness in patients, even though they received the flu shot

Answer 1

Natural selection led to evolution of viruses. For example flu virus.

Viruses have high mutation rates, so when viruses are replicating genetic mutation may occur that led the virus to be weaker or may be stronger. When mutation allows the virus to become stronger, then by natural selection it will survive more successfully.

During flu season, the strand of flu which was in the flu shots, was not successful because strand that was made for the vaccine has changed.

Flu viruses continuously changes through a natural process called as antigenic drift. But degree of antigenic drift and the frequency of drifted viruse may vary for seasonal flu vaccine.

Therefore during flu season even though person received the flu shot led to flu illness in patients.