Vaccine expert Dr. Peter Hotez called out the "wackadoodle" harassment he has suffered after podcast host Joe Rogan issued a $100,000 challenge for him to debate Democratic primary contender Robert F. Kennedy Jr. on vaccine misinformation.

"Let's face it, when you have RFK Jr. and Joe Rogan and Elon Musk all tag-teaming, those 'tres hombres' at the same time, I mean, that probably includes just about every follower on Twitter. ... It was pretty overwhelming," Hotez told MSNBC's Mehdi Hasan.

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Hotez has been subjected to harassment since the challenge was first made, he said.

"The attacks from the anti-vaccine lobby do come in waves, and this time, though, it's been one of the tougher ones because ... physical stalking and the incessant threatening emails and the stuff online. It's just total wackadoodle."

The feud started after Hotez called out Rogan for allowing Kennedy Jr. to come on The Joe Rogan Experience last week and reiterate his beliefs regarding the COVID-19 vaccine and vaccination in general.

Among other claims, Kennedy reiterated a story about a mother who came to him with an autistic son who changed his outlook on vaccination, suggested media figures silenced comedians who questioned the vaccines, and detailed how pharmaceutical corporations profited from distributing the vaccine.

Debating RFK Jr. on Rogan's show would not serve anyone's benefit, according to Hasan.

"Set aside that Joe Rogan wouldn't even be a neutral moderator of such a debate. He's endorsed anti-vax nonsense many times before," he said. "But, you just can't, in general, debate with conspiracists and loons."

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"You can't debate whether up is down, hot is cold, black is white. They never change their minds, and they win just by you agreeing to share a platform with them."

Hotez debating RFK Jr. with Rogan would be akin to wrestling a pig, according to Hasan. "You both get dirty, and the pig gets 40,000 new followers on Twitter."