Flies in the order Simuliidae, known as “black flies,” such as this fly in the genus Simulium, can carry blood parasites that cause the malaria-like bird disease leucocytozoonosis. While the parasites can’t survive the human immune system to cause disease, bites from black flies can still be painful and cause unpleasant reactions in humans, such as fever, headache, nausea, and swollen lymph nodes. (Photo © Charley Eiseman, via BugGuide.net, CC BY-ND-NC 1.0)