An outbreak of Legionnaires' disease in the South Bronx has killed two people and sickened dozens more, and New York City health officials are urging anyone with symptoms of the lung infection to promptly seek medical attention.

But wait, wait, wait. What is Legionnaires' in the first place?

From 8,000 to 18,000 people are hospitalized each year in the US with Legionnaires' disease, according to the Centers for Disease Control and Prevention. It's an infectious disease, but unlike other things like influenza and the common cold, it doesn’t spread from person to person; people catch it by inhaling mist from water systems harboring Legionella bacteria. The first known outbreak occurred in 1976, at an American Legion convention in Philadelphia, which is where the name comes from.

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Symptoms can show up two days to two weeks after someone is exposed, and can include fever, cough, chills, muscle aches, headache, fatigue, confusion, loss of appetite, and diarrhea.

“It is a true bacterial pneumonia. That’s why it is severe, but at the same time it doesn’t make everyone ill, and it doesn’t kill everybody,” Hassan Bencheqroun, MD, an interventional pulmonologist and critical care specialist at Pacific Pulmonary Medical Group, and an assistant clinical professor at the University of California, Riverside, explained to Health. “Those that develop the pneumonia and those that die from it are those that have risk factors.”

These risk factors include:

  • Being 50 or older
  • Having a chronic lung condition such as asthma or emphysema
  • Smoking cigarettes
  • Taking medications that suppress the immune system
  • Having an immune-suppressing illness

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Legionella pneumophila is the strain responsible for 90% of cases of Legionnaires' disease. The bug lives in the condensers of large air conditioning systems, hot water tanks, whirlpool spas, cooling towers, even ornamental fountains. While low levels of the bacteria won’t lead to an outbreak, “it’s when it multiplies to the level that it would cause disease that we worry,” Dr. Bencheqroun explained.

Legionella is usually a rarer cause of pneumonia,” Belinda Ostrowsky, MD, MPH, infectious disease attending physician and director of antibiotic stewardship at Montefiore Health System in the Bronx, added in an interview with Health. “In the Bronx we can see clusters (increased number of cases) in the summer months.  Clusters can also be seen at times due to specific environmental sources.”

Investigators have honed in on cooling towers at the region’s busiest hospital and a mall/movie theater complex as the source of the current outbreak underway in the Bronx. To confirm that these structures are the outbreak source, medical sleuths must match the DNA signature of the bacteria in the towers to the bacteria that’s making people sick.

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Death rates from Legionnaires' disease range from 5% to 30%. Some patients will get better with just a few days of intravenous antibiotics, while others may need to spend ten days to two weeks in the intensive care unit, according to Dr. Bencheqroun. “Most of these people have an incredible amount of inflammation in the lining of their lungs, so their oxygen level is very low,” he says. Some people may simply need some extra oxygen, while others may need to be on a ventilator.

The good news, Dr. Bencheqroun says, is that inexpensive, very effective drugs for treating Legionnaires' disease are available. “We have excellent antibiotics that treat this pneumonia, and these antibiotics are not complex, third tier, only-accessible-to-the-rich kinds of antibiotics,” he said. “Most of the cases of Legionella pneumonia that we diagnose, we send home, and they restart their life with just a story to be told.”

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