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 synthesizing the personal and the universal to help all of us through this new frontier.