The perfect human pathogen?

As I reported earlier, a whole lot of people including me, fell ill over the holiday break with what was very likely a norovirus infection. Carl Zimmer has just written a fascinating article on the remarkable family of noroviruses, based on an paper published in a recent issue of The Journal of Infectious Diseases by Aron Hall of the Centers for Disease Control. Zimmer writes:

Each norovirus carries just nine protein-coding genes (you have about 20,000). Even with that skimpy genetic toolkit, noroviruses can break the locks on our cells, slip in, and hack our own DNA to make new noroviruses. The details of this invasion are sketchy, alas, because scientists haven’t figured out a good way to rear noroviruses in human cells in their labs. It’s not even clear exactly which type of cell they invade once they reach the gut. Regardless of the type, they clearly know how to exploit their hosts. Noroviruses come roaring out of the infected cells in vast numbers. And then they come roaring out of the body. Within a day of infection, noroviruses have rewired our digestive system so that stuff comes flying out from both ends.

The virus uses extremely devious methods to spread itself by infecting as many others as possible.

Once the norovirus emerges from its miserable host, it has to survive in the environment. Noroviruses have no trouble doing so, it seems. Fine droplets released from sick people can float through the air and settle on food, on countertops, in swimming pools. They can survive freezing and heating and bleaching. In 2010, scientists surveyed a hospital for noroviruses and found 21 different types sitting on a single countertop. It takes fewer than twenty noroviruses slipping into a person’s mouth to start a new infection.

It would be very nice if we only had to worry about getting noroviruses once and then could enjoy protection from them for the rest of our lives. Unfortunately, it seems that we only have a brief protection of perhaps a few months, and then we’re fair game again. As a strain of norovirus encounters this short-lived defense, it evolves new ways to evade our immune systems. A modified strain can then sweep around the world in as little as three months.

The norovirus is a great example of one of the features of evolution where one organism increases its own survival value by manipulating other organisms (in this case humans) to aid in its own reproduction and spread, thus increasing its selection advantage.

Although the virus made our whole family miserable for several days, I have to admit a grudging respect for such an ingenious and durable little beast.


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