In order to fly an airplane, a person is required to hold not only a pilot licence that matches the airplane and flight operation being conducted, but a valid medical certificate. In Canada the licence is currently a bluish bit of high quality paper, embossed and indented with lines so pilots can easily fold it to fit in our wallets. The medical is a similar document printed on grey-beige paper. In the past, the medical was known as a licence validation certificate, a very accurate term, because without it, a pilot licence is not valid. At regular intervals, depending on age, pilots must visit a specially qualified doctor, a Certified Aviation Medical Examiner or CAME to undergo an examination that renews the medical. If a pilot has any any temporary incapacity--e.g. the flu or a broken ankle--she should not fly. She might consult a CAME if there is any doubt.
A pilot I know is approved for a Category 1 (commercial) medical even though he takes medication to control his blood pressure. One morning while driving his car to the airport, he momentarily greyed out and lost control of the car. He went off the road, driving up over the curb and onto a suburban lawn. He succeeded in stopping the car without hitting the gentleman who was on that lawn, watering flowers.
The gardener was a doctor. But not just a doctor, the gardener was a CAME. But not just any CAME. My acquaintance had blundered woozily onto the lawn of the head of civil aviation medicine for the entire region, one fifth of the country.
The pilot had had a recent change in his blood pressure medication and his doctor had neglected to give him an associated restriction on flying and driving. Didn't his doctor know he was a pilot? But of course. In fact, his personal doctor happens to be the head of civil aviation medicine for the region. That's whose lawn he was on.
He's back flying now, and he gave me permission to tell this story.