A silent epidemic of chronic illnesses afflicts tens of millions of Americans. Renowned writer Meghan O’Rourke delivers a revelatory investigation into this elusive category of “invisible” illness that encompasses autoimmune diseases, post-treatment Lyme disease syndrome, and now long COVID, synthesizing the personal and the universal to help all of us through this new frontier. Drawing on her own medical experiences as well as a decade of interviews with doctors, patients, researchers, and public health experts, O’Rourke reveals how inherited ideas of cause, diagnosis, and treatment have led us to ignore a host of hard-to-understand medical conditions, ones that resist easy description or simple cures. And as America faces this health crisis of extraordinary proportions, the populations most likely to be neglected by our institutions include women, the working class, and people of color.