Read about Marek's disease. https://en.wikipedia.org/wiki/Marek%27s_disease https://en.wikipedia.org/wiki/Marek%27s_disease "However, the leaky vaccine changes

Read about Marek's disease. https://en.wikipedia.org/wiki/Marek%27s_disease https://en.wikipedia.org/wiki/Marek%27s_disease "However, the leaky vaccine changes this evolutionary pressure and permits the evolution of highly virulent strains.[13] The vaccine's inability to prevent infection and transmission allows the spread of highly virulent strains among vaccinated chickens."