Dr. Begely: So, this seems to be an animal virus, and not a plant one.

Dr. Shelly: No, wait! What about viroids? Couldn't they cause the disease too? We can't just eliminate the possibility that it's not a virus just because it didn't infect the plant!

Is it possible that this thing could be a viroid?

 Well, I'm pretty sure it's not a viroid.

 Yes, it's definitely a viroid. That would explain the strange way it only infected the animals.