City health officials are warning residents about an outbreak of Legionnaires' disease in the Bronx.

The warning comes after a recent spike in cases throughout the borough. The city's Department of Health says 11 patients contracted the infectious disease in December. Dr. Judy Berger, of St. Barnabas Hospital, says the spike is unusual.

Legionella -- or Legionnaires' disease -- is a bacteria which comes after someone contracts pneumonia. Health officials say the disease usually comes 2-10 days after someone is exposed to an environmental source.

That source is usually aquatic organisms, which usually grow in warm environments - including standing water and faucets.

Doctors say inhalation of contaminated aerosols from devices like cooling towers and air conditioners are sources where people can contract the disease. Health officials say the highest rate can be found in northeast Bronx neighborhoods.

Doctors at St. Barnabas Hospital say anyone that contracts pneumonia and goes to the hospital should ask the physician to also test for Legionnaires' disease.