https://www.greenmedinfo.com/blog/coronavirus-contagious
It was Pasteur who convinced a skeptical medical community that contagious germs cause disease; his "germ theory" soon became and still serves as the official explanation for most illness. However, in his private diaries, he stated unequivocally that in his entire career he was not once able to transfer disease with a pure culture of bacteria (and he obviously wasn't able to purify viruses at that time). In fact, the only way he could transfer disease was to either insert the whole infected tissues into another animal--he would sometimes inject ground-up brains of an animal into the brain of another animal to "prove" contagion--or resort to adding poisons to his culture, which he knew would cause symptoms in the recipients.
Eventually, Pasteur admitted that the whole effort to prove contagion was a failure, leading to his famous deathbed confession that "the germ is nothing, the terrain is everything."19 In other words, microbes may be associated with diseases but they do not cause them; the cause is always something that has upset the natural order of nature, such as nutrient deficiencies, toxins, bad air, filthy water, electromagnetic radiation or even the emotions of fear and despair.