When a host dies with the pathogen still inside the pathogen dies as well. Resistance to invaders evolved as a result of the development of the mechanism of immunity. The development of immunity depends on the recognition of differences in chemical structures of substances. The first requirement of an immunological system is to be able to recognize substances which are foreign to the host. These substances, known as antigens, stimulate the immune system by producing antibodies.
Immunological protection is dependent on repeated stimulation.
Isolated communities may elude exposure to an infection for years. Such is the case with the New World. The New World inhabitants had no previous exposure to the Old World disease and therefore carried no immunity for them. Once a disease is introduced from locality to another outbreaks will occur. Europeans, having already encountered these pathogens, acquired immunities over the years due to prolonged interaction.