Anti-Malarial Mosquitos

Malaria used to be a big killer in tropical areas. The virus was borne on the ubiquitous mosquitos, who transferred it from person to person by biting them for some blood, to help lay their eggs. After the mosquito bit one infected person, they would spread the infection to hundreds of others. This majorly slowed European exploration of Africa, Oceania, and South America, because exploration parties almost inevitably developed severe malaria, and medical treatment was unavailable. Large amounts of exploration parties died in the jungle. Often the survivors were so sick that they had to unceremoneously leave their collegues where they fell.
But Malaria is a virus, not an inherent result of mosquito bites. Mosquito bites in North America, where I live, almost never spread malaria. And this gave a biologist a big idea. Discovery news reports that biologists are breeding mosquitos that kill the virus in their guts, thus changing from a disease-spreading vector to the remedy for that very disease. The idea began by noting that even in wild mosquitos, their immune system fights the virus. The mosquito is not a sadist. It doesn't bite you to irritate you, it bites you because a dash of your blood allows it to lay way way more eggs. (Mammals like humans are like fertility clinics to mosquitos, and the virus is like radical social darwinists who burn down fertility clinics.) So the genetically engineered mosquito has beefed up immune and digestive systems. The virus is more likely to be ground up to serve as a free protein for the mosquito, and if it survives that, a powerful immune system will likely tear it apart before it comes within a mile of a human. This will make malaria much less present, and save millions of humans from malaria misery.
As is in the tropics, it's necessary to have a cache of malaria-treating medicine on hand, and an insect-proof screen to sleep under at night, because without one, one will be bitten perpetually, and infection is inevitable. With these new mosquitos in the pool, mosquito bites will only mean itchiness, not horrible disease with a risk of death.