@vbhide They vaccinated chickens to fight Marek's disease (which was benign at the time). The vaccinated chickens rarely got sick but they were still propagating the virus.
Normally, in nature, deadly viruses have trouble propagating because they kill their hosts. What happened with Marek's disease is that the vaccinated chickens propagated the deadliest strains.
Now, all chickens have to be vaccinated at birth because Marek's disease has become worse than Ebola.
https://www.pbs.org/newshour/science/tthis-chicken-vaccine-makes-virus-dangerous