Zoonotic viruses are the types of pathogens that often learn to co-exist with their natural hosts peacefully. This means that besides being infected, the animal carrying the virus do not necessarily develop the disease. Viruses, similarly to all living organisms, strive to survive and reproduce (spread). When a big chunk of their primary hosts get disease and die, the virus have difficulties surviving as well, since all viruses can’t exist by themselves – they always need a living host.
Therefore, the species which carry the viruses under investigation as their primary hosts are called natural reservoir.