He became a physician at the age of 14 (the pilot has him turning 16), due to taking an interest in medicine after having survived leukemia twice.
Falling ill due to the damage done to the environment, Mother Nature was placed in the care of Physician Doogie Howser and several other doctors at St. Terra Hospital in the town square of Anytown during April 22, 1990.
Arriving in the hospital to check on her was Dr. Emmett Brown who borrowed Doogie's tie to set off the keyboard circuits in his briefcase which in turn activated a display screen showing the current environmental disasters around the world, as well as mentioning that the last of the rainforests were gone by 2057.
Though the other doctors were skeptical of those claims, Doogie took them seriously and was interested in hearing him out. As such, he was held responsible for any further visits by Emmett.