If, as a physicist, I were to be told that quarks have never been isolated, and so they are not actually real particles, it would take some effort to respond. I'd have to talk about the Standard Model and Asymptotic Freedom, and thus admit that they do have some properties that are not like other particles.
If faced with insistance that this proves the case, that they are actually some sort of epiphenomenon of particle accelerators, I might have some difficulty knowing how to proceed. It's true that they manifest an as yet new characteristic, and so perhaps we have to expand our conceptualizaiton of what a particle is.
That may not be a perfect analogy for my question, but I have a medical friend who would like to tell me that no virus has ever been isolated, and that as opposed to some kind of transmissible, infectious organism, they are actually some kind of waste product of cells. I am told that any attempt to satisfy Koch's postulates involves the injection of materials in addition to the viruses at issue, and the claim is that it's these combination of materials that is making the animal sick.
I see a lot of varied "viruses don't exist" claims around the web, focused on HIV, measles, and CV19, and associated with names like Drs. Stefan Lanka, Andrew Kaufman, Tom Cowan, etc. Associated claims are that in any context that involves supposed exposure to a virus, less than 100% of the animals get sick, and so whatever may be going on, it's the animal itself at issue. In some quarters this is called "Terrain Theory." I would like to not have to dig too far into this.
How can I respond in a coherent way that doesn't require becoming a virologist?