Badge of Infamy - Lester del Rey

Daniel Feldman was a doctor once. He made the mistake of saving a friend's life in violation of Medical Lobby rules. Now, he's a pariah, shunned by all, forbidden to touch another patient. But things are more loose 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 humanity on two planets, Feldman finds himself a pivotal figure. War erupts. Earth is poised to wipe out the Mars colony utterly. A cure to the plague is the price of peace, and only Feldman can find it. (summary by Steven H. Wilson)