Knock, knock.
Who's there?
Covid.
Covid who?
The thing that killed half a billion people!
Listen up, buttercup. This joke is about COVID, which was like, a super bad flu that went around and made a bunch of people super dead. The punchline is dark because it reminds us that COVID killed a LOT of people, and you, my friend, probably still wear a mask in your car. Sad!