[calcapt]

Just some thoughts. First of all you are not sure what caused the captains incapacitation so you have to assume it could be anything. First thing, put on your oxygen mask and establish communications with ATC, informing them of your situation and declaring an emergency. If oxygen deprivation or airborne agents caused the problem with the captain you certainly don't want to be exposed to the same problem. Get help immediately from any available resource and get the captain out of the way. Your responsibility now as PIC is not the captain, it is the safe operation of the flight until it's conclusion. Your number one priority is to safely fly the plane. Get a FA upfront and tell him or her what you want done. Have them check for pilots in the back as well as medical personnel and have the FA inform the passengers that the capt is ill. You haven't got time to do that. You have declared a medical emergency and you are proceeding to the safest, not necessarily the closest airport. Stay in the FO seat as that is the seat you are most comfortable sitting and landing in. Stop the airplane on the runway centerline and make it accessible to the medical personnel by shutting down engines, setting the brakes and whatever else your particular aircraft requires. Take time to slowly think about what needs to be done as decisions made in haste are often wrong. Hopefully the captain will recover from whatever put him down and he will buy the beer on the next layover.