If I remember correctly, disease in the past started going down right before vaccines started coming out for them, which is related to the general increase in sanitation in the same time period. Are vaccines more necessary now due to stronger strains that exist due to the defenses we put up?
See Marek's disease; where what once was a mild disease in chickens turned deadly after it strengthened in response to vaccines. Chickens basically have to be vaccinated against it now, lest they die.
See Marek's disease; where what once was a mild disease in chickens turned deadly after it strengthened in response to vaccines. Chickens basically have to be vaccinated against it now, lest they die.