The Grand Tour host has revealed that he was diagnosed with Covid-19 over Christmas and does not know if he has fully recovered.
Writing in his column for The Sunday Times Clarkson said: "Four days before Christmas, I woke in the night to find my sheets were soggy. And that I had a constant dry cough."
After his test came back positive, "the doctor was very clear - I'd feel under the weather for between five and 14 days and then I'd either get better or I'd have to go to hospital.
"Because I am 60 and fat, and because I've smoked half a million cigarettes and had double pneumonia, I'd probably die, on my own, in a lonely plastic tent."
The Who Wants To Be A Millionaire? added: "I also had to self-isolate... I took myself off to bed with the new Don Winslow book and a bag of kale to wait for the Grim Reaper to pop his head round the door.
"I'm not going to lie - it was quite scary.
"With every illness I've had, there has always been a sense that medicine and time would eventually ride to the rescue.
"But with Covid-19 you have to lie there, on your own, knowing that medicine is not on its way and that time is your worst enemy."
While he "didn't feel too bad" initially, Clarkson's "breathing really did start to get laboured, and there was always the doctor's warning ringing in my head about how it might suddenly get worse".
He said: "We keep being told that we know a great deal about Covid, but what I've learnt over the past 10 days is we don't.
"We don't know how long we are infectious for. We don't know how to tackle it. We don't know what it does to us.
"We don't know how long the antibodies last. We don't know how easy it is to catch it twice. And we certainly don't know if any of the vaccines will work long-term. I don't even know if I'm better now."