There have been more than 100 reported cases of Legionnaires’ disease in New York City since July 10, 2015, and 12 of those infected with the disease have died. The outbreak has been traced to the cooling system of the city’s historic, century-old Opera House Hotel, in the South Bronx. Health officials believe the outbreak has reached its peak, and the hotel has announced that it will test its cooling towers every 30 days—three times as often as required by law.

But what is Legionnaires’ disease, exactly, and how could an air conditioner cause such a massive outbreak of infection?

The disease, and the bacteria that causes it, got its name after a 1976 outbreak among attendees of an American Legion conference in Philadelphia. Out of 2000 conference guests, 221 people were infected and 34 died. At the time, the cause was unknown, and it took the Centers for Disease Control (CDC) around six months to isolate and identify the pathogen.

Legionellosis is any infection of a group of bacteria called Legionella. The most infamous of these infections is the atypical pneumonia called Legionnaires’ disease, which is usually caused by a strain called Legionella pneumophila. The good news is that most people who are exposed to L. pneumophila or its less-frequently seen siblings L. longbeachae, L. feeleii, L. micdadadei, and L. anisa don’t usually get sick. And though infection can be deadly if you do contract Legionnaires’ disease, it’s treatable.

While there are between 8000 and 18,000 people hospitalized with Legionnaires’ disease each year in the U.S., most healthy people can and do recover through treatment with antibiotics, though there is a risk of lifelong complication from an infection left untreated too long. People most at risk for a serious or deadly outcome are immunocompromised, over age 50, smokers, or suffering from chronic lung illnesses such as chronic obstructive pulmonary disease (COPD).

The sooner treatment begins, the better the chances are of recovery. One reason treatment might be delayed is that Legionnaires’ disease’s symptoms—high fever, cough, shortness of breath, muscle aches or headaches—are similar to other respiratory illnesses, such as influenza or other types of pneumonia. While the symptoms usually appear two to 10 days after exposure, it can take as long as two weeks to become symptomatic.

Fortunately, Legionnaires’ disease cannot be transmitted from person to person. Instead, the bacteria is spread aerobically, so people are infected by breathing in airborne mist from warm, contaminated water, such as can be found in improperly cleaned hot tubs, ventilation systems, or large plumbing systems. In the 1976 case, the bacteria was eventually found in the cooling towers of the Bellevue Stratford Hotel where the conference took place, and it was rapidly distributed throughout the building via the air conditioning system—a scenario almost identical to that of the current NYC outbreak.