During a six-month period, 239 cases of pneumonia occurred in a town of 300 people. a clinical case was defined as fever 39°c lasting > 2 days with three or more symptoms (i. e., chills, sweats, severe headache, cough, aching muscles/joints, fatigue, or feeling ill). a laboratory-confirmed case was defined as a positive result for antibodies against coxiella burnetii. before the outbreak, 2000 sheep were kept northwest of the town. of the 20 sheep tested from the flock, 15 were positive for c. burnetii antibodies. wind blew from the northwest, and rainfall was 0.5 cm compared with 7 to 10 cm during each of the previous three years. the situation is an example of