Severe Acute Respiratory Syndrome (SARS) spreads so quickly and is so hard to treat, despite being related to the common cold, because it's formed from a rare combination of mammalian and avian viruses. That makes it unrecognizable by human immune systems.
Geneticist David Guttman found that the proteins on the left side of the virus comes from mammals, such as cats, cows and mice, while the proteins on the right come from birds, such as chickens and ducks. The middle part of the virus is a mix of both.
Our immune systems would usually recognize a coronavirus of bird origin, as most flu is, and start fighting it immediately. However, the part of it that originated with mammals allows it to sneak past our immune systems.
Viruses often mutate, but this type of genetic change is more dangerous than most. Guttman says, "These recombination events have the potential to create an entirely new structure essentially instantaneously. Since our immune systems have never seen this new viral form, it is more difficult for them to respond to it in a timely and effective manner." He thinks that other flu viruses that hit especially hard in the past may have also been hybrids, such as the 1918 Spanish Influenza that killed over 20 million people.
We now know that SARS was originally transmitted to humans from civet cats sold in southern Chinese food markets, but it's not known how it picked up the bird virus. "It's possible that a civet picked up the virus from a bird," says Guttman. "This could have created the opportunity for a very rare recombination event that produced a virus with a new host range. Basically, the recombinant virus is infectious to humans, while the two parent viruses are not. This new virus likely then spread to humans due to poor hygiene and close quarters in the food markets of Southern China."
With regard to HIV, one of the most deadly viruses ever to infect humanity, it's been discovered that some people have a gene mutation which makes them more resistant to getting AIDS, despite having HIV. These people can carry the HIV virus in their bodies for years without developing symptoms. Since these people are of European ancestry, this is another reason why AIDS has become such a plague in Africa, where the gene mutation is not present. A new study by the University of California at Berkeley shows that the ancestors of people with this genetic mutation got it by surviving smallpox.
About 10% percent of Europeans have this mutation, which arose 700 years ago. Earlier speculation was that these were people who had survived bubonic plague, but researcher Alison P. Galvani says, "Our population genetic model finds that genetic selection from plague wouldn't have been sufficient to drive the frequency of this genetic mutation to its current level. [However,] it was sufficient for smallpox." People with this mutation would also be more likely to survive smallpox today, at a time when terrorists are threatening to use it as a bioweapon.
Bubonic plague hasn't been a major cause of death in Europe for 250 years, but smallpox was only eradicated in 1978, giving people more chances to develop the mutation. Also, most of the people exposed to smallpox were infected before the age of 10, and the disease's 30% mortality rate killed off those who were vulnerable to it, so they didn?t live long enough to reproduce. Those who survived to adulthood had inherited the gene mutation, which they passed along to their offspring.
"The Scandinavian countries in particular have very high frequencies of this [mutation]?14 to 16%--which some people have taken to mean that Vikings dispersed [it]," says Galvani. "But it could also be due to smallpox hitting Scandinavian countries harder. There were certainly some big smallpox epidemics in Scandinavia, whereas plague affected the continent more, in particular Italy and France."
The mysteries of medicine mean that people who survive SARS today may have relatives in the distant future who are able to survive a deadly virus that comes along at that time.
When it comes to the body, sometimes we just want to get away from it all.
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