The Medic removes all the cubes of the same color, not only 1, when they use the Treat Disease action.
If a disease has been cured, the Medic automatically removes all the cubes of that color from a city, simply by entering it or being there. This does not require any action.
The Medic's automatic removal of cubes can occur on another player’s turn, for example, when the Medic is moved by the Dispatcher or an Airlift Event.
It is also impossible to place cubes for a cured disease (for whatever reason) in the same city as the Medic.