Katherine J. Wu

On its face, reinfection appears to be a straightforward term. It is literally “infection, again”—a recovered person’s second dalliance with the same microbe. Long written into the scientific literature of infectious disease, it is a familiar word, innocuous enough: a microbial echo, an immunological encore act.But thanks to the pandemic, reinfection has become a semantic and scientific mess.

To locate some of the world’s most superpowered cells, look no further than the human immune system. The mission of these hometown heroes is threefold: Memorize the features of dangerous microbes that breach the body’s barriers. Launch an attack to bring them to heel. Then squirrel away intel to quash future assaults.The immune system is comprehensive, capable of dueling with just about every microbe it meets.