How does migration of human populations affect epidemics?

When people move from one place to another, they spread disease. If a population that lacks immunity to a certain pathogen moves into an area where the resident population has immunity, the incoming population may suffer an epidemic. Bringing new people into an area can introduce new pathogens to an unsuspecting population with no natural immunity. For example, yellow fever and African strains of malaria reached the New World via the African slave trade.