According to his agent, Nielsen passed away at the age of 84 due to complications from pneumonia.
Bit of a personal observation here: A number of years ago I had the opportunity to interview Mr. Nielsen, and during the conversation we discussed the complexities of what seemed like the silly type of comedy he had mastered. As an example, he pointed to one of the Naked Gun movies in which his character, Frank Drebin, is marrying Priscilla Presley's character.
"Originally," he explained to me, "the idea was that O.J. Simpson's character would end up tied up in the tin cans attached to the car and dragged behind us as we drove away. But we realized that that was too obvious; that the audience would have already figured it out. They knew that something bad always happens to O.J. when the two of us are together..."
I cleared my throat and nervously, well aware of the bad taste of the question, asked, "Uh...did you happen to be in Brentwood with him when... you know....."
Without hesitation, Nielsen let out a loud guffaw, and then quickly cut himself off, admonishing both of us, "I shouldn't even be laughing at that."
But he did. No doubt your response is, "He laughed at that? Surely you're joking."
I'm quite serious, and don't call me Shirley.