Why can mosquitoes carry deadly viruses, such as West Nile and dengue, without succumbing to them? The prevailing theory maintained that the viruses and mosquitoes evolved to live in harmony. But entomologists have found quite the opposite to be true. They infected mosquitoes with a test virus and found that the mosquito's immune system cut up the pathogen's genetic material so that the insect did not get sick. In contrast, when the mosquitoes were given a genetically modified version that blocked the gene-chopping mechanism, the insects could not mount an attack against the invader and died off more than four times as quickly. The discovery might lead to antivirals fashioned to mimic the mosquito's virus-killing tricks. The Proceedings of the National Academy of Sciences USA published the findings online December 1, 2008.