It's not useful to try and explain an illness by a singular cause, when multiple factors are extant and important. Susceptibility to flu is influenced by immun

It's not useful to try and explain an illness by a singular cause, when multiple factors are extant and important. Susceptibility to flu is influenced by immune system fortitude; that's uncontroversial. That stress is one factor (among several) that can weaken the immune system in an otherwise healthy person, is surely useful to consider when making life choices.