Looking back, Offit said he was sandbagged. "He's a liar. He lied about who he was, he lied about what he was doing. He was just wanting to set me up," Offit said.

Though that was the only time they have ever spoken, Kennedy has continued to disparage and malign Offit over the years. In his book dedicated to denigrating Fauci, Kennedy spends plenty of time spitting insults at Offit, calling him a "font of wild industry ballyhoo, prevarication, and outright fraud." He also makes the wildly false claim that RotaTeq "almost certainly kills and injures more children in the United States than the rotavirus disease."

Inconvincible

Understanding that Kennedy is a germ theory denialist and terrain theory embracer makes these attacks easier to understand—though no less abhorrent or dangerous.

"He holds these beliefs like a religious conviction," Offit said. "There is no shaking him from that," regardless of how much evidence there is to prove him wrong. "If you're trying to talk him out of something that he holds with a religious conviction—that's never going to happen. And so any time anybody disagrees with him, he goes 'well, of course, they're just in the pocket of industry, that's why they say that.'"

There are some aspects of terrain theory that do have a basis in reality. Certainly, underlying medical conditions—which could be considered a disturbed bodily "terrain"—can make people more vulnerable to disease. And, with recent advances in understanding the microbiome, it has become clear that imbalances in the microbial communities in our gastrointestinal tracts can also predispose people to infections.

But, on the whole, the evidence against terrain theory is obvious and all around us. Terrain theorists consider disease a symptom of an unhealthy internal state, suggesting that anyone who gets sick is unhealthy and that all disease-causing germs are purely opportunistic. This is nonsense: plenty of people fall ill while being otherwise healthy. And many germs are dedicated pathogens, with evolved, specialized virulence strategies, such as toxins, advanced defense mechanisms, and antibacterial resistance. They are not opportunists.