evidence: The COVID-19 pandemic has breathed new life into a 160-year-old rivalry. Science skeptics are flocking toward a fringe set of beliefs called "terrain theory," an ideology that ranges from total denial of the existence of viruses and bacteria to the belief that lifestyle choices alone force otherwise benevolent microbes to transform into pathogens. The central gist is that the body’s "terrain," not germs, creates disease—reasoning that offers legitimacy to skeptics of masks and vaccines. In the Facebook group "Terrain Model Refutes Germ Theory," which has grown from 147 followers to 20,700 since the beginning of the pandemic, one member who tested positive for COVID-19 speculated that he fell ill because he broke his ankle. Another member argued that "measles is a developmental cleansing." [...] It didn’t take the rivals long to jump from chemical reactions to illness, and that’s where their ideas veered in different directions. When Pasteur began to develop a theory of infectious disease, Béchamp called it "the greatest scientific silliness of the age," according to the same article in the South African Journal of Science. In response, he developed what he later called "terrain theory," which looked quite different from the modern movement. He believed that microorganisms are essentially benign, and that pathogens emerge when structures inside our cells, called microzymes, transform into bacteria in response to unhealthy environmental conditions—like tiny Dr. Jekylls transforming into Mr. Hydes. In other words, he believed that disease causes pathogens, and not the reverse.