Mukherjee begins by noting that, before the COVID-19 pandemic, “it seemed as if the immune system, of all the complex cellular systems in the body, was the one that we understood the best” (245). Researchers were creating drugs that helped make cancer visible to T cells. A fifth of all drugs being approved by the US Food and Drug Administration were related to the immune system. Then came the novel coronavirus, SARS-COV2, which resulted in the COVID-19 pandemic and revealed huge gaps in the understanding of the human immune system. The pandemic brought wealthy and less wealthy countries alike to their knees and had caused at least six million deaths at the time Mukherjee wrote this book.

At the beginning of the pandemic, researchers didn’t understand “what feature of the interaction between SARS-COV2 and human cells enabled this virus to precipitate a global pandemic” (249). A German clinic was one of the first to start uncovering clues when they realized two important things: that asymptomatic/presymptomatic individuals could transmit the virus and that the rate of transmission was exponential. In addition, the world collectively realized thenpredicttable nature of the virus: Some people had mild infections, while others had lethal infections.

The pandemic taught scientists that there’s still much “we don’t know” (253) about the immune system.