Actually there are three families of virii that are capable of infecting plants, animals and humans.
I know it's rather foolish to argue about a fictional premise such as this as most fans of the show don't care about scientific facts of reality. But I simply can't suspend my disbelief because all it takes is an eighth grade understanding of biology to figure out that this premise is a joke, even by scifi standards.
At any rate, regarding your "3 viruses that affect humans", you can find anything you want to "support" your argument using a
cursory Google search. But when you go more in depth you discover that hair-brained theories are just that, hair-brained theories. There is no evidence that plant viruses can jump to humans and vice versa. Sure there's one small "study" about a virus in peppers that a handful of participants
anecdotally claimed made them feel bad. There was no medical examination of their physiological response, just a casual mention that a handful of people in this study "felt bad" after eating peppers. That's hardly proof that plant viruses can infect humans. (
Infect as in enter the host and begin to reproduce.)
And when one does a Google search for the subject of plant viruses
infecting humans one can't find any history of such a thing. Human biology simply doesn't support it. At most one can infer a possible allergic reaction to toxins produced by plant viruses, but that is not the same as infection. As it is humans are allergic to many botanical substances, but again, that's not infection and it's not lethal pandemic-ally. If humans started to croak from eating tomatoes word would quickly spread and the populace would stop eating tomatoes and people would stop dying. The virus in those tomatoes would not be passed from the humans who ate those tomatoes to humans who eschewed "love apples" (old moniker for tomatoes). There would be no pandemic because an allergic reaction is not the same as a viral infection. Neither is poisoning, i.e. having a (lethal) reaction to toxins in the consumed plant material. And those toxins would most likely be created by the plant in reaction to the virus infecting it, not by the virus itself.
I can't expound on this point enough. Any reaction that humans would have to plant viruses would be in regards to toxins that are consumed when plant material is ingested. And as I said, those toxins would be produced by the plant in response to its viral pathogen. The toxins would not be from the virus itself (as opposed to bacteria or fungi which can excrete toxins).* So basically what you would have is simple poisoning, not infection. At best you would have an immune response based upon an allergic reaction. Remove the offending allergen and the response ends because there is no replication of any viral pathogen to continue the assault upon our immune system. Plant viruses simply cannot replicate once in the human body. Our biology doesn't allow for it. Allergen yes, pathogen no.
*And even if I'm wrong regarding viruses not excreting toxins it's a moot point. Once those plant viruses entered the human body they would die off because they could no longer reproduce as humans don't have plant physiology. A better story premise for this show would be having the virus infect human gut bacteria, which we all need to live. But then again, how do you make the jump to infecting the world's flora as the same logically-deduced problem would exist regarding cross-species (cross-phylum?) infection.
*I was tempted to use the word "viri" or "virii" as a designation for virus but couldn't corroborate it as being legit via a Latin or Greek root. My trusty Latin/Greek root word dictionary had "virus" and other off shoots but the closest to viri was "vir" which means "a man". I'm not saying that "viri" isn't acceptable in modern usage, but I'm just pointing out that it piqued my interest to see if it was the proper use of the singular, or plural, version of "virus". Some Google sites said no and some said yes, go figure. Incidentally, my father, who was a highly trained botonist/horticulturalist, gave me that root word dictionary. He used it for the botany and biology classes he taught. It's a great reference book to have on hand and is much better than the Internet when looking up the origins of words I have found.
*Here's a link with a more scientific explanation of what I am getting at regarding my criticism of the premise of this show. It talks about a tobacco virus affecting humans but is clear to point out that plant viruses don't replicate inside the human body. It also mentions the "pepper study" and clearly states that plant virology isn't compatible with human biology.
http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0060621
