When people think about infectious diseases — as many have, these last three years — they think mainly about the immune system. The severity of an individual’s illness, it’s assumed, is down to how well the immune system detects, attacks and eliminates the pathogenic invader.

The immune system is said to resist disease. Resistance reduces the amount of pathogen residing inside a host, thereby curtailing disease progression, driving recovery or preventing infections altogether. People who are immunocompromised fear infections because they cannot effectively resist pathogens.