Linked from
The 12 pages that link to Natural reservoir, each with the reason it gives.
ZoonosisRelated: Animal reservoirs maintain pathogens that can eventually infect people.
HostRelated: A host can maintain a parasite even when other susceptible populations are absent.
Infectious disease transmissionRelated: Reservoirs maintain pathogens between transmission events.
Wildlife diseaseRelated: Wildlife reservoirs can maintain pathogens that also infect other species.