The number of new cases of a certain disease within a certain population. The total number of cases at any time within a certain population. The percent of cases that end in death. For example, rhinoviruses cannot survive outside of humans. At any point in time there are always some people infected.
Humans are the reservoir of rhinoviruses.
Many opportunistic infections are derived from normal microflora- a huge reservoir of microbes. Pathogens with a human reservoir may be eradicated by vaccination efforts. For example, the smallpox virus was maintained in a human reservoir. Worldwide vaccination efforts lead to eradication of this disease in part because smallpox cannot survive outside of humans. Some human pathogens also infect animals.