During an encephalitis outbreak in Chicago, a scientist searches for its carriers in a watery tunnel. Although most mosquito-born illnesses have been virtually wiped out in North America, the United States is still vulnerable to the viral disease, which was responsible for 150 human deaths and 2,000 illnesses in 1975.

(Text adapted from and photograph shot on assignment for "Mosquitoes, The Mighty Killers," September 1979, National Geographic magazine)