At first glance, the cause of severe infectious disease seems obvious: The culprit is a bacterium, a virus, or some other pathogen. Yet this can’t be the full story. After all, there are plenty of differences in a given pathogen’s effects from one person to the next, notes Jean-Laurent Casanova, a human geneticist of infectious disease at Rockefeller University. This clinical variability, he says, “is the fundamental infection enigma.”
Various factors contribute. Another disease or condition may have made the body more vulnerable, or the immune system may have weakened with age. And maybe one person got a larger dose of the pathogen than another did.