In theory measles could be eradicated - like smallpox it has no animal host.
This third way is to insert their genes into another, more conventional virus instead of a traditional animal host.
And crucially, like polio, smallpox had only one animal host: Homo sapiens.
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That experiment was based on the long-held notion that a pandemic strain is more likely to emerge when a flu virus mixes its genes with another virus in an animal host, such as a pig.
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We now know that the hot-springs that sustain these "vent" ecosystems occur in every ocean basin and are host to hundreds of previously unknown animal species.
Until researchers can determine what animal is the natural host of the virus, and how MERS spreads from the host to humans, each new outbreak is dangerous and mysterious.
Although few pathogens can survive in both an animal and a human host, there is laboratory evidence that resistance can be passed across the species barrier through meat products.
But no one knows which disease will be next, what animal will be its natural host, where it will happen, or how dangerous it will be.
Waiting out there could be some profound discoveries about human and animal behavior, epidemiology and a host of other sciences.
But it is not simply a matter of an animal virus suddenly finding humans to be a congenial host, and flourishing as a result.
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It is also now thinking of banning the gambling halls which play host to the "numbers game", a Brazilian form of street bingo based on animal names which is hugely lucrative for those who run it.
Occasionally, one of these animal viruses is able to jump into humans - either directly or through an intermediate host, such as a chicken or a swine.
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