The German endured an inconsistent campaign at Stamford Bridge, but finished on a high by scoring the winning goal against Manchester City in the Champions League final.
"The illness hit me hard," Havertz said ahead of Germany's Euro 2020 last-16 clash with England at Wembley.
"It lasted some four to five weeks before I could even think of getting back on to a football pitch. During the time I was ill, I was quite ill, to be honest. And I wish it upon absolutely no one.
"Then the aftermath was difficult, too. I really had to work hard to get back to the pre-Covid fitness levels. You have to live with it."
Havertz also revealed how the lockdowns in London inhibited his ability to settle in his new home city.
He added: "I haven't really explored London a lot or as much as I would have liked to.
"It really is difficult to do anything in terms of leisure activities. Every three days you have an important match coming up, so there is precious little time to go sightseeing or go for a meal.
"It looks a bit bleak but that's the way it is. You come home from training and are happy about every hour that you can spend inside your own flat.
"I do hope that later, London will give me much more joy, and I'll have many, many good London experiences."
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