evidence: There is plenty of proof that viruses exist and cause disease. Flu viruses have been photographed in the process of infecting cells, under powerful microscopes. And their RNA, their unique genetic information, has been sequenced and studied. [...] Béchamp’s hypothesis was that pathogens were harmless unless the body or "terrain" they occupied was ill. This is why its supporters call it "terrain theory". Terrain theory is incorrect, but has attracted plenty of believers during the Covid-19 pandemic.