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).

As Kennedy describes in his book, his preferred theory “emphasizes preventing disease by fortifying the immune system through nutrition and by reducing exposures to environmental toxins and stresses,”—not using vaccines or advanced medicines to fight off specific pathogens as suggested by germ theory.

Denying germ theory

While no respected health expert would argue against the benefits of a healthy diet and an environment free of toxic substances, this does not negate the reality that obligate pathogens, with evolved and specialized molecular weaponry to invade and ravage their victims, cause specific diseases. Still, Kennedy disparages germ theory and attempts to undermine it with shaky arguments.

In the hearing on Wednesday, Sanders called attention to Kennedy’s denial of germ theory while raising one of Kennedy’s shaky arguments for debunking. In opening statements, Sanders warned Kennedy that he wanted to question the “things that you have written which call in doubt the very existence of the germ theory.”

During his questioning, Sanders referenced a passage in Kennedy’s book that reads:

A doctrinal canon of the germ theory credits vaccines for the dramatic declines of infectious disease mortalities in North America and Europe during the twentieth century. … Most Americans accept this claim as dogma. It will therefore come as a surprise to learn that it is simply untrue.

Sanders pointed out a 2024 study led by the World Health Organization and published in The Lancet that found that since 1974, vaccines had saved an estimated 154 million lives, including 146 million children under the age of 5—or, as WHO put it, vaccines saved the equivalent of six lives every minute of every year over the past 50 years.

“My question is a simple one,” Sanders said, “do you still believe that one of the central tenets of the germ theory, that vaccines sharply reduce infant mortality, is quote-unquote simply untrue?”

Kennedy responded first by trying to discredit the WHO study, noting that it was based on modeling. Using a common tactic of anti-vaccine advocates, he instead redirected to one of his preferred, cherry-picked studies, which was a 2000 study in the journal Pediatrics with lead author Bernard Guyer. The study, “Annual Summary of Vital Statistics: Trends in the Health of Americans During the 20th Century,” also included estimates and algorithms in its analysis.

Kennedy’s argument in the hearing—and his book—is that improvements in nutrition, hygiene, and sanitation led to large declines in infant and child deaths during the 20th century—not vaccines. One might argue that improvements in hygiene and sanitation leading to fewer childhood deaths would be a supportive argument in favor of germ theory. It is well established that waterborne pathogens in contaminated drinking water and sewage cause life-threatening diseases. But to Kennedy and his anti-vaccine allies, it supports the idea that removing environmental toxins allows for a healthier internal terrain, preventing disease, with the added benefit of making vaccines seem unneeded.

Debunking claims

The study by Guyer notes that sanitation, among other public health strategies introduced in the first half of the 20th century, drove major declines in mortality. But, as Cassidy noted during the hearing, it’s not all that the study found. Cassidy looked up the studies Kennedy raised and read through them during the hearing.

The Guyer study highlighted that vaccination did not become widely used until after the middle of the century, thus it cannot account for mortality declines prior to that. But it concluded, as Cassidy read out loud at the hearing:

The reductions in vaccine-preventable diseases, however, are impressive. In the early 1920s, diphtheria accounted for about 175,000 cases annually and pertussis for nearly 150,000 cases; measles accounted for about half a million annual cases before the introduction of vaccine in the 1960s. Deaths from these diseases have been virtually eliminated, as have deaths from Haemophilus influenzae, tetanus, and poliomyelitis.

Kennedy also brought up another study by John and Sonja McKinlay, which was published in the Milbank Memorial Fund Quarterly. (The Milbank Memorial Fund is notable for providing nearly 40 years of funding for the infamous and shameful Tuskegee syphilis study, for which it publicly apologized in 2022.)  The McKinlays’ 1977 article was titled “The Questionable Contribution of Medical Measures to the Decline of Mortality in the United States in the Twentieth Century.” It concluded that “In general, medical measures … appear to have contributed little to the overall decline in mortality in the United States since about 1900.”

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