disease cluster
n. A geographical region where an above average percentage of the population has, or has had, a particular disease.
Every time a disease cluster turns up, communities worry, scientists scramble for a cause and, as in the new movie based on Jonathan Harr's 1995 book, "A Civil Action" (Random House), about a leukemia cluster in Woburn, Mass., lawyers start suing.
—Gina Kolata, “Probing Disease Clusters: Easier to Spot Than Prove,” The New York Times, January 31, 1999
An unexplained pattern of illnesses has been found in hundreds of people living near the nation's nuclear facilities, The Tennessean in Nashville reports today.

Peter Brush, DOE's top health official, . . . said DOE officials will review the national health complaints raised by The Tennessean. If disease clusters are found, the DOE could urge the CDC to begin a study.
—Kathryn Winiarski, “News report traces pattern of illnesses near nuclear plants,” USA Today, September 29, 1998
1981 (earliest)
Medical sleuths troubled by an outbreak of meningitis that has killed four people and afflicted 30 others in Houston are seeking a common source for a puzzling cluster of the disease in the nation's fifth-largest city.
—Sharon Herbaugh, “Doctors Seek Common Link To Puzzling Disease Cluster,” The Associated Press, February 07, 1981