Quick: what's the scariest animal? Sharks? Grizzly bears? Alligators? Nope. In terms of sheer deadliness, nothing can hold a candle to mosquitoes, as this great chart from Bill Gates' personal blog shows:
To be fair to the tiny insects, almost all of the killing here is done by infections spread by mosquitoes (predominantly malaria), rather than the mosquitoes themselves. But they're very, very good at spreading those infections, and many of those infections kill with alarming frequency.
That's why Gates is throwing a "Shark Week"-style "Mosquito Week," publishing articles on what it's like to have malaria, raising dengue-resistant mosquitoes, and more. If you're interested in ways we could conceivably eradicate mosquito-borne illnesses altogether, be sure to check out Andy Mills' fantastic Radiolab segment on biotech entrepreneur Hadyn Parry's efforts to genetically engineer mosquitoes carrying genes that will cause their offspring to die well before reproducing themselves, a scheme which Parry claims is capable of killing off 80 percent of the mosquitoes in a given area in a matter of months. There are less dramatic options in that vein as well, like engineering mosquitoes with immune systems strong enough to kill the parasite that causes malaria before it can spread to human.
Crazy genetic engineering schemes like that are probably a number of years from being viable at a large scale, and in the meantime, distributing anti-malarial bednets is probably the most cost-effective way to fight the illness. The Against Malaria Foundation is a particularly good charity in that area, if you're looking for a place to donate.