In the past, when a swine unit wanted to eradicate a certain disease such as Pseudorabies, costly depopulation/repopulation was the only viable solution. Depopulation, facility cleanup, and repopulation is not always successful because nearby herds with a lower health status my provide a source for reinfection. In 1980, Tom Alexander of Cambridge University developed a system for producing pigs 'free of certain diseases without costly depopulation.