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Avian influenza is an excellent example of this. High pathogenicity avian influenza (HPAI) strains such as H5N1 and H7N9 have repeatedly spilled over into human hosts. Luckily, to date, there has not been a subsequent sustained human-to-human transmission in the spillover events from the current strains of HPAI; when human-to-human transmission has occurred, it has been limited.
Moreover, should a natural zoonotic virus spillover into a human host, subsequent human-to-human transmission is often not possible or unsustainable. Most spillover events are dead ends, meaning that the infectious agent could infect a novel host but is not capable of sustaining a transmission cycle within that new host. This happens frequently, particularly in areas where human and zoonotic populations overlap.
Spillover events are very rare, but the chances of them occurring can differ between viruses and hosts. Viruses can occasionally, but quite rarely, move from one species to another. With so many variables, such as complex pathogen life cycles, modes of transmission, and random susceptible hosts’ interaction, an initial spillover event resulting in ongoing transmission into a new host is relatively rare and usually an imperfect process. Most mammalian viruses lack the ability to infect humans.
Sad to hear Science in Action is ending ... we’ll done Roland