Most investigations into disease clusters are busts. But every now then, researchers do find clusters. Here are several noteworthy ones:

Asbestos: In the 1960s, researchers traced the development of mesothelioma to exposure to asbestos, which was used heavily in shipbuilding.

Legionnaire's Disease: An outbreak of pneumonia at a convention in 1976 in Philadelphia claimed 34 lives. Investigators traced the cause to bacteria in the hotel's air conditioning system.

Liver cancer: A study in 1974 of liver cancer contracted by four workers in a Kentucky factory led to the identification of vinyl chloride as a carcinogen.

Charleston: In 1998, residents in the industrial Neck Area asked DHEC to investigate whether there were excess cancers in their neighborhood. Using a new cancer registry, researchers traced pleural cancers to asbestos used at the old Navy shipyard.

From the Centers for Disease Control and Prevention:

A suspected cancer cluster is more likely to be a true cluster, rather than a coincidence, if it involves one or more of the following factors:

--A large number of cases of one type of cancer, rather than several different types.

--A rare type of cancer, rather than common types.

--An increased number of cases of a certain type of cancer in an age group that is not usually affected by that type of cancer.

--Myasthenia gravis is a chronic autoimmune neuromuscular disorder that hits people's voluntary muscle groups.

--Scientists don't think it's inherited or contagious.

--People with myasthenia gravis are thought to have antibodies that attack connections between nerves and muscles, creating muscle weakness.

--Common symptoms include drooping eyelids, blurred vision, slurred speech, difficulty swallowing, weakness in arms and legs, breathing difficulties.

--There is no cure, though some medicines can help alleviate the condition