Bear in mind that I am not a biologist.

As I understand it, Marek's disease is a highly transmittable disease in chickens that used to be fairly mild. A vaccine exists, but only prevents symptoms and not whether the chicken actually contracts the virus.

The thing is, viruses are naturally driven not to evolve to kill their hosts. It's bad for procreation, obviously. If you kill your host before you can multiply and spread to other hosts, you're ineffective as a virus. Viruses are naturally selected to be HIGHLY transmissible and MINIMALLY deadly. But remember, what about a virus kills the host? It's not the virus itself, it's what the virus causes: symptoms. The very thing the vaccine for this disease prevents. And if you prevent symptoms, the virus is free to evolve into the most horrifying, deadly virus possible. Normally, a virus evolving in this way would quickly die out, because the deadly symptoms would kill its host preventing transmission and replication. But now? The vaccinated chicken doesn't care: it can't get symptoms and thus cannot die to the virus. But it can be infected because the vaccine doesn't prevent infections.

And so what do you think happens the instant this non-symptomatic, armageddon virus carrying chicken gets near a flock of unvaccinated chickens? Complete and utter genocide, effectively.

It ends in a system where chickens have to be vaccinated for a disease that used to be common course and relatively non-lethal because some fucking retard made a vaccine that doesn't prevent infection, only symptoms and God help anyone who doesn't see the relevance of the bolded statements to current human world affairs.