A landmark exploration of one of the most consequential and mysterious issues of our time: the rise of chronic illness and autoimmune diseases. A silent epidemic of chronic illnesses afflicts tens of millions of Americans: these are diseases that are poorly understood, frequently marginalized, and can go undiagnosed and unrecognized altogether. 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. Drawing on her own medical experiences as well as a decade of interviews with doctors, patients, researchers, and public health experts, O'Rourke traces the history of Western definitions of illness, and 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.