Unusual influenza viruses keep making headlines. A virologist explains how they decide if something is worth fretting about.
In the US, a dairy-farm worker develops itching, blood-shot eyes. In Australia, a young girl falls ill after a foreign holiday and is rushed to hospital. In Mexico, another man, already ill and bed-bound, becomes seriously unwell and dies. Each of these recent cases was caused by a different strain of influenza virus. In each case, it was an animal virus, which should not normally have turned up in humans at all.
How far has it got? This is usually the first question. It’s actually really hard for a virus to adapt to growing well in a new host species. Even influenza viruses – basically bird viruses, but notorious for causing repeated human pandemics – only manage it every few decades.
We can do this because viruses mutate quickly. Lining up the differences in their genetic sequences lets us build family trees , which we can use to reconstruct how the virus got to particular places at particular times. What else can we look for? We worry more about viruses jumping between similar host species, because this is easier for the virus to do. Influenza that’s already in a mammal is closer to being able to infect us than influenza from a bird.
Could it get worse? Adapting to humans is hard for a virus, so anything that gives a virus more chances to pull this off is a concern. Sustained outbreaks are more of a risk than one-off cases. And we worry about people doing anything that gives a virus more chances to get used to them. Things like drinking unpasteurised milk in areas where it could carry H5N1 influenza viruses, for example.
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