Badge of Infamy, by Lester del Rey

An interesting premise but the execution and the final resolution were disappointing.

Daniel Feldman, disgraced doctor, flees from the Medical Lobby and ends up on Mars. Why is he disgraced? Because he dared to defy the Medical Lobby by treating a patient outside of an official hospital. He saved the patient's life, by the way, but violations of Medical Lobby rules of ethics allow no wiggle room.

On Mars, the Medical Lobby rules technically still apply, but the locals don't care so much. Feldman ends up doing clandestine medical work for the settlers while the local Mars authorities look the other way.

An outbreak of Martian plague raises the stakes considerably. Feldman not only does medical work, but he starts doing medical research as well. He discovers that the plague has a fourteen-year incubation period and has likely infected most of Earth as well as Mars. Its threatens humanity with extinction. During all this, the Martians settlers rebel against the Earth authorities and start a war.

The answer ends up being ludicrously simple: smoking the local Martian tobacco alternative, bracky weed, kills the parasite responsible for the plague. And since it will take at least a decade for Earth to isolate and synthesize the particular compound in bracky weed that will stop the plague, Earth immediately capitulates and grants Mars indepedence. In exchange, Mars will supply enough bracky weed to cure everyone.