Diagnosing and Treating Medicus Incomphensibilis is a book of case studies on revising medical writing into plain English. It is a companion to Plain English for Doctors and Other Medical Scientists (Oxford University Press, 2017). It gives more practice to help the reader master skills in plain English medical writing. The 12 case studies are based on excerpts from articles published in leading medical journals. The excerpts cover a wide range of medical topics. Each case study looks at one excerpt, between 56 and 308 words long, that shows many classic symptoms of medicus incomprehensibilis - those overused writing habits that make medical writing hard to read. The case study asks questions, and gives short exercises, to guide the reader through the process of diagnosing the symptoms of medicus incomprehensibilis. The reader writes their prescription and revises to treat the symptoms. After each case study, the authors give their answers, prescription and revision. This book is intended for doctors and other medical scientists who write for medical journals, and anyone who aspires to do so. It is intended for writers at all levels, from veteran authors to students. It includes writers in related fields such as public health, pharmacology, nursing and life sciences. It is designed for self-study, seminar or classroom use.