The singer, 48, said it was "touch and go for a couple of weeks" and choked back tears as he thanked his doctors "from the bottom of my heart".
Speaking outside his home in London after being discharged from hospital in Vienna, the former Wham! frontman became breathless after just a couple of minutes.
He revealed he had been suffering from streptococcus pneumoniae, a form of pneumonia, and had had to undergo a tracheostomy.
Michael said: "I feel amazing. I mean, I'm very weak but I feel amazing. I'd say touch and go for a couple of weeks but we played it down.
"I didn't want to worry my fans too much and I'm really sorry I couldn't contact them in any way before now but I was really not in any state to.
"They spent three weeks keeping me alive basically. I don't want to take you through all of it because some of it I want to protect my family from and I'm sure I'll get it all written down but it was by far the worst month of my life.
"I'm incredibly, incredibly fortunate to be here and incredibly fortunate to have picked up this bug where I did.
"Apparently the hospital in Austria that they rushed me to was absolutely the best place in the world I could have been to deal with pneumonia so I have to believe that somebody thinks I've still got some work to do here."
George MichaelThey spent three weeks keeping me alive basically... It was by far the worst month of my life. I'm incredibly, incredibly fortunate to be here.
Michael had started his tour in Prague in August and played 45 shows in 35 different cities before falling ill.
He was also due to play a series of UK dates up to the week before Christmas.
The singer became so ill that he had to be treated in intensive care.
He promised fans they would have tickets for another performance and also said he would like to play for the doctors in Austria who "spent three weeks keeping me alive".
"I spent the last 10 days since I woke up literally thanking people for saving my life which is something I have never had to do before and never want to do again," he said.
I'm back and I'm proud, though not very loud...... (which, I assure you, will be temporary!) MERRY XMAS TO ALL OF YOU! ISN'T LIFE GRAND !! X December 23, 2011
Michael credited his family, fans and the "the fact that I still have plenty to live for" for helping him to pull through the illness.
He added: "I have an amazing, amazing life and if I wasn't spiritual enough before the last four, five weeks then I certainly am now."
The singer, wearing dark glasses and wrapped up in a scarf and coat, still looked unwell and spoke haltingly - saying that he was still trying to get over the tracheostomy.
He said: "I'm not supposed to speak for very long, but merry Christmas" and rejected photographers' pleas to take off his glasses.
Michael later posted on Twitter: "I'm back and I'm proud, though not very loud...... (which, I assure you, will be temporary!) MERRY XMAS TO ALL OF YOU! ISN'T LIFE GRAND !! X."
