IOWA CITY —
So, Why Aren't We All Dead?
With so many potentially deadly organisms lurking in the animal species we share the Earth with, the question becomes not if we'll have another novel pandemic, but when. However, these events — the new influenzas, the SARS coronaviruses, the HIV outbreaks — are actually relatively rare. "Spillover" events — an individual becoming infected with a zoonotic pathogen — are common, but typically the infected host is a dead end. He or she doesn't spread the germ to a second person, which is necessary for an epidemic (which is a localized outbreak) or a pandemic (a worldwide infection) to occur. Going back to H5N1 influenza versus H1N1, that's why the former has caused only sporadic outbreaks and the latter has become pandemic. H1N1 is readily transmissible between people, and H5N1 (so far) is not. This is also why there was so much concern earlier this year when a genetically modified H5N1 was created in a laboratory setting. This modified virus was able to spread readily between ferrets, a common animal model for human influenza research. The work caused worry that such a virus may escape from a lab and spread in the wild — "The Stand" come to life.
This controversy also highlights the difficulty in studying potential zoonotic pathogens. Many of these organisms have adapted to their hosts and do not always cause symptoms in their "natural" species. As such, it's difficult to anticipate which microbes will 1) make the species jump successfully; 2) cause illness in the new host species (for example, in humans); and 3) transmit efficiently among members of the new host species. Prediction right now is very foggy, though we're beginning to better understand the diversity of organisms out there, and with that, hopefully gain understanding into why some spill over and others do not.