I don't understand your reference. More info? Search turns up nothing useful.
It’s a disease in chickens. It didn’t used to be fatal but it was expensive for commercial producers because some of the chickens that survived it developed cancerous tumors and could not be used. So a vaccine was developed. However, it was an imperfect vaccine — it didn’t stop transmission. The virus ”learned” to escape the vaccine, which resulted in a
much more lethal version. Last I read, there are five wild strains, the worst of which has been 100% fatal in every unvaccinated bird they tested it with. Any unvaccinated chicken exposed to that strain will die, including those bred on small farms who might otherwise forego the vaccine.
(Sorry if someone else already explained.)