That well known source of truth, the internet, has it that Marat (French revolutionary assassinated in his bath by Charlotte Corday in 1793) suffered from coeliac disease. This is posted on Wikipedia, as well as in a few other places.
This is an interesting suggestion, though I can’t find a reference anywhere to the research that suggests that he had coeliac disease. It is believed that he liked to bathe often in cool water to alleviate symptoms of a skin disease, and it is this skin disease that has led people to suggest coeliac disease.
Except … coeliac disease is primarily a disease of the intestine, not of the skin. Perhaps Marat had Dermatitis Herpetiformis (DH), which produces an intense and unbearably itchy rash which is alleviated by a gluten free diet?