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Soleh vaccinates his chickens against newcastle disease virus (NDV). A month later he introduces some unvaccinated chickens into his flock. Another farmer in the area has not vaccinated his chickens and when many of his birds get sick and die the veterinary investigation identifies NDV. Soleh's birds did not get sick even though one of the sick birds from his neighbour's farm had mixed with Soleh's chickens. Having a high proportion of vaccinated birds in his flock almost certainly produced a herd immunity effect that protected both the vaccinated and unvaccinated birds in Soleh's flock from infection with NDV.