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According to Health Department officials, a hospital in Pittsburgh has sent 4,700 letters to patients who may have been exposed to tuberculosis.

The warnings were issued after it was discovered that an employee at the hospital tested positive for the disease. They say that the likelihood of actually contracting the disease is very low.

Most people infected with the bacteria that cause tuberculosis don’t have symptoms. When symptoms do occur, they usually include cough (sometimes blood-tinged), weight loss, night sweats, and fever.