Not that I am aware. I think it is more that a very, very small percentage show an immunity and they form the breeding stock and then succesive generations will become succeptible again when the virus mutates or they loose the susceptibility by generation - I dont know what the exact mechanism is. I did a bit of research on it way back when it was released as I was covering the story for some newspapers and magazines. Havent kept up with the science of it since then and I am sure much has been learnt. It sure is a very successful virus.