Badge of Infamy by Lester Del Rey

As a doctor, Daniel Feldman once made the mistake of saving a friend's life in violation of Medical Lobby rules. Now he's a pariah, shunned by all, and forbidden to touch another patient. But rules are looser on Mars -- there, Doc Feldman is welcomed by the colonists, even as he's hunted by the authorities. But when he discovers a Martian plague may soon wipe out two planets, Feldman finds himself a pivotal figure. War erupts, Earth is poised to completely destroy the Mars colony, and a cure to the plague is the price of peace.