| If you read the Wikipedia page on Koch's postulates you'll see that Koch himself abandoned and modified them for various bacteria. https://en.wikipedia.org/wiki/Koch's_postulates#Postulates Viruses, and some unculturable bacteria and other organisms, are simply unique with regard to Koch's second postulate, in that they literally can't be cultured in a pure manner. Hierarchical and tangential flow filtration can, however, get them incredibly pure (as quality controlled by electron microscope) after culturing. I'm not an expert here. This is probably not routinely done as it would be costly and to no purpose for the particular experiment. But it can be done, and I'd be extraordinarily surprised it it hadn't been done. > We've never, again as far as I'm aware, isolated a virus from a sick person, exposed a healthy person to it, and seen the same disease symptoms show up with the virus now present in their system. Yes, we also don't typically expose healthy people to disease-causing bacteria either. This is a hard sell post-WWII. Typically when you culture a virus you'll also have a separate culture that is exposed to everything else minus the virus. You'll "purify" both in the same manner, expose some animals or cultured cells to the purified virus culture, and expose other animals or cultured cells to the purified control culture. The only thing that differs is the presence of the virus. You can also literally view cells being infected by physical viruses in culture, and image or video what happens to them during this process. E.g. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6330349/ Read the "Routine Diagnosis" part of this article. |