Scientists have created a genetically engineered mosquito breed that can’t carry the malaria-causing parasite, an innovation that could help stop a disease that kills a million people a year. The breakthrough involved tweaking just a single bug gene, one involved in making insulin. After making the change, researchers pumped 90 of the engineered insects with the Plasmodium parasite; 10 days later, when normal mosquitoes would have guts full of the malaria-causing parasites, researchers found zero. A 100-percent-effectiveness rate is unprecedented, and the results shocked the scientists, who were only hoping to cut down on the parasites. Blocking just 90 or 95 percent of the parasites could cause them to mutate, much like how overused antibiotics can lead to drug-resistant bacteria. The perfect blockage rate is a happy accident; researchers had only hoped to shorten the mosquitoes’ lives to give the malaria parasite less time to mature.