Houseflies aren't the most charismatic of insects, but be thankful they aren't tsetse flies. Though roughly similar to a housefly in appearance (and shown in close-up here, just to make you squirm), tsetse flies come equipped with a large proboscis, which they use to feed on the blood of large vertebrates. (Yes, that includes humans).
That appetite for blood makes them a vector — meaning they transmit disease, in this case trypanosomiasis or sleeping sickness. It's estimated that as many as 300,000 people are infected with this debilitating disease, which results in swelling of the brain.