An endemic is the constant presence of a disease in a limited geographical area. It differs from the term epidemic, which is the occurrence of an infectious and contagious disease which expands rapidly within a certain population, but disappears, and the term pandemic, which is a widespread outbreak. Thus, in some foreign countries where there is an endemic, a vaccination against certain diseases is required before visiting them. For example, it is advisable to be vaccinated against yellow fever before visiting Africa or South America, or to take a preventive treatment against malaria in the tropics. People living in contact with these diseases are generally immune, but its presence remains constantly contaminating part of the population.