Mark Cuban—Shark Tank star, billionaire entrepreneur, and owner of both the Dallas Mavericks and Cost Plus Drugs—waded into a chaotic Twitter brawl about vaccines on Sunday involving some big names—including Tesla CEO Elon Musk, the world’s richest man.

It centered around Spotify podcaster Joe Rogan challenging a medical expert—Dr. Peter Hotez, dean of the National School of Tropical Medicine and professor of pediatrics and molecular virology and microbiology at Baylor College of Medicine—to enter a debate about vaccines on his show. That came in response to Hotez sharing a Vice article from Friday entitled “Spotify Has Stopped Even Sort of Trying to Stem Joe Rogan’s Vaccine Misinformation.”

The Vice article came after a Rogan episode the day prior featuring Robert F. Kennedy Jr., a long-shot Democratic presidential candidate, political scion, and—like Rogan—high-profile vaccine skeptic. Despite his political party, U.S. conservatives have taken a shine to Kennedy, who wrote a 2021 book entitled The Real Anthony Fauci: Bill Gates, Big Pharma, and the Global War on Democracy and Public Health. He’s also frequently deplored the war in Ukraine, calling it a ”money laundering racket for military contractors” on the podcast.

Rogan tweeted on Saturday evening, “Peter, if you claim what RFKjr is saying is ‘misinformation’ I am offering you $100,000.00 to the charity of your choice if you’re willing to debate him on my show with no time limit.”

“Joe, you have my cell, my email, I’m always willing to speak with you," replied Hotez, who wrote the 2018 book Vaccines Did Not Cause Rachel's Autism: My Journey as a Vaccine Scientist, Pediatrician, and Autism Dad.

Twitter owner Musk then tweeted, “He’s afraid of a public debate, because he knows he’s wrong.”

Hotez replied, “Seriously Elon? This is monstrous. 200,000 Americans needlessly perished (including 40,000 Texans, our neighbors) because they were victims of antivaccine disinformation during our awful Covid delta/BA.1 waves in 2021-22. Please don’t do this…”

Hedge fund billionaire Bill Ackman unexpectedly joined in, tweeting, “I will add $150,000 to @joerogan’s wager so now $250,000 can go to charity and the public can hear an open debate on an important topic.”

Tom Nichols, a staff writer at The Atlantic, tweeted about the invitation: “No medical professional should ever agree to do this. Never. It elevates the conspiracy guy, demeans the medical professional, and will only convince the kooks out there that RFK is right because a real doctor took the time to debate him.”