I heard this saying sometime ago, but I wasn't completely sure about it. Is that any research/info that support this saying or you all have a different opinion about this?
It's really an interesting topic. I am also curious about it, but it's really hard to say. You must know that the etiology of fish diseases are poor, there are some agents need to be classified. As I know there was a novel virus was detect on the salmon's gills published on JoV . And my personal opinion is that we need to classified agents caused diseases is really tough and there are not sufficient resources for us to do research on the rare pathogens, because some of them are couldn't cause big loss on aquaculture. Maybe there are some pathogens we don't know but caused diseases now. I would like to hear from others on this topic.
Literature suggested that 54.9% bacterial pathogens, 22.6% viruses, 3.1% mycotic agents, and 19.4% parasitic agents are responsible for periodical disease outbreaks in fish cultures (Dhar et al., 2014).