Had a Flight Engineer take a fit on a red eye. (B727). About 0300, he complained of back pain earlier, but nothing to drastic. Myself and the F/O were present on the flight deck, and it would have to be said, it was quite spectacular. He lost complete control of his body functions, he fell sideways only kept in by his shoulderstrap, and he had no control of his limbs which were all over the place. I hit the button for the Cabin Manager, who thankfully was a trained nursing sister, she assessed him straight away, and he slowly came out of it, and had no memory of it what so ever, in fact just went back to his panel like nothing had happened. Needless to say the F/O and I just looked at him dumbfounded. Turned out he had a raging kidney infection, which pushed his blood pressure up thru roof, which in turn caused him to take a fit. I realised then what we take for granted (good health) is not always so, and enrolled myself in a first aid course, in the hope it would give me some insight to medical problems on the flight deck. Luckily for us many of the female flight attendants at TAA were trained nursing sisters, and were a great help in illness in both PAX and crew, (and the occasional death).