By Katie Reilly

NEW YORK (Reuters) - A deadly outbreak of Legionnaires' disease, a severe type of pneumonia, has now killed four people and sickened 65 in the Bronx section of New York City since July 10, New York City health officials said on Saturday.

This wave of Legionnaires', which officials have called unusual, is now more than five times the number of cases recorded in the last outbreak, in which 12 people in the Bronx fell ill in December 2014.

The disease is caused by Legionella, a bacteria found in certain plumbing systems, including hot tubs, humidifiers, cooling towers and hot water tanks. It is spread by breathing in mist from water, and cannot be spread from person to person.

The illness is most common in the summer and early fall, according to the Centers for Disease Control and Prevention. Symptoms include fever, cough, headaches and muscle aches.

In response to the outbreak, the city's health department has inspected 22 buildings in the Bronx, 17 of which have cooling towers. Five buildings, including the historic Opera House Hotel, Lincoln Medical Center and the Concourse Plaza mall and movie complex, tested positive for Legionella.

Disinfection efforts are ongoing or have already been completed at all five sites.

Julio Vargas, general manager of the Opera House Hotel, said the water in the cooling tower used by the hotel has been treated and disinfected, and there have been no reports of hotel guests falling ill.

The people who died from the disease were older adults with underlying medical problems, according to a city press release.

The department said the city's drinking water supply, fountains and pools have not been affected.

The disease earned its name following a 1976 outbreak among people who attended a Philadelphia convention of the American Legion, a veterans organization.

Between 8,000 and 18,000 people are hospitalized with the disease each year in the United States, according to the CDC.

(Reporting by Katie Reilly; Editing by Diane Craft)