The answer is an onboard medical emergency that sometimes requires an unscheduled landing. A recent New England Journal of Medicine study looked at about 12,000 inflight emergencies over a three year period. Pregnancy complications, psychiatric episodes, and cardiac arrest were relatively rare. More common were fainting, difficulty breathing, stomach problems and cardiac symptoms.
The research showed some interesting statistics. Passengers who are physicians provided help in almost half the cases, nurses helped in 20%, and emergency medical technicians in 4%. Unscheduled landings occurred in 7% of cases with 9% admitted to a hospital and .3% died.
(Did You Know?  I assisted in an in-air emergency. My story below.)
The University of Pittsburgh provides a 24-hour service for in-flight consultations. So when there is a suspicion of someone having difficulty this service is called. They then obtain a history and get vital signs from the flight attendants. If needed, they ask for physicians or medical staff on board for further assistance.
In the instance that I assisted, the person was medically stable when I got there. My presence served two-fold. First was if the patient worsened I was there to begin CPR or whatever needed. Secondly, and perhaps most importantly, to provide reassurance to the person. Because as my patients have taught me time and again, they are not a medical problem but are human beings.
(from Melinda Beck at the WSJ)
