In a Congressional hearing on Wednesday, Sen. Bernie Sanders (I-Vt.) directly confronted anti-vaccine Health Secretary Robert F. Kennedy Jr. on his rejection of germ theory—the unquestionable scientific idea that specific pathogenic microbes cause specific diseases. After Kennedy defended his fringe view, Senator Bill Cassidy fact-checked and debunked Kennedy’s denialist arguments in real time.

The exchanges mark a rare instance in which Kennedy’s dismissal of germ theory has been raised in such a high-profile public setting, in this case, a hearing of the Senate Committee on Health, Education, Labor, and Pensions. Kennedy, who has no background in science, medicine, or public health, is well known as an ardent anti-vaccine activist and peddler of conspiracy theories. But his startling rejection of a cornerstone theory in biomedical science has mostly been underreported.

As Ars Technica reported last year, Kennedy wrote about his germ theory denialism explicitly in his 2021 book The Real Anthony Fauci. In it, Kennedy maligns germ theory as a tool of pharmaceutical companies, scientists, and doctors to promote the use of modern medicines. Instead of accepting germ theory, Kennedy promotes a concept akin to the discarded terrain theory, in which diseases stem not from germs, but from imbalances in the body’s inner “terrain.” Those imbalances are claimed to be caused by poor nutrition and exposure to environmental toxins and stressors. (In his book, Kennedy erroneously labels this as “miasma theory,” but that is a different theory that suggests diseases derive from breathing bad air, vapors, or mists from decaying or corrupting matter. The idea was supplanted by germ theory, while terrain theory was never widely accepted.)

Kennedy’s embrace of terrain theory over germ theory is foundational to the priorities of his Make America Healthy Again (MAHA) movement, which promotes notions of healthy diets and lifestyles and clean living. As health secretary, Kennedy has focused on revamping federal dietary guidance, focusing on whole foods (and concerning amounts of saturated fat) while vilifying artificial ingredients and additives. He has regularly posted videos of himself working out on social media. And, with his previous career as an environmental lawyer, he has a long history of fighting against environmental contamination. Kennedy and his MAHA movement have strongly lobbied against chemical pollutants and pesticides, particularly glyphosate (though Kennedy notably shifted on this issue recently, and now supports increasing production of the weed killer, in line with Trump’s policies).