It started with a sudden outbreak of illness at a Philadelphia hotel in 1976. The victims were overwhelmingly American Legion member, who were gathered for an annual conference, that year swelled by Bicentennial fervor. The party screeched to a halt as dozens of attendees were sickened.

As George Dehner describes in his article on the history of the disease, “the affliction, dubbed Legionnaires’ disease in commemoration of its 29 unlucky fatal victims and their 153 stricken colleagues, struck suddenly and left the attending physicians mystified about its cause.”

It would eventually be identified by scientists at the CDC. It’s unusual in being named not for its location of emergence (like Spanish flu, Rocky Mountain Fever, Norwalk virus), its symptoms (scarlet fever), or its vector (cat scratch fever), but its first known victims, the Legionnaires.

It soon became clear, however, that the reveling Legionnaires weren’t the first to be sickened by this bacterium. Examining old samples at the CDC, scientists showed that Legionella had been responsible for a respiratory disease outbreak among patients at St Elizabeth’s hospital in Washington, DC, in 1965 and for an outbreak at the Oakland County Public Health Department in Michigan in 1968. This—less deadly—wave of illness had been named “Pontiac Fever.”

Once they’d identified the culprit, scientists realized that Legionella are everywhere.

“The bacterium has been recovered from standing and running freshwater courses, soil, and even from rainwater puddles,” writes Dehner. “If one looks for it, one can find it.”

If it’s all around us, why was it only in the late twentieth century that it became a public health threat? Dehner tells us that Legionnaires’ is “generally grouped in the category of ‘emergent diseases’ that are new, newly discovered, or resurgent organisms that are causing illness.” But most “emergent diseases” are part of conversations about privation, poor hygiene, malnutrition, and overcrowding. As Dehner describes,

a typical example cites the conditions of the desperate slums that surround the fast-growing megacities of the world. These urban areas have served as magnets for migrants from rural areas, crowded together in neighborhoods that have outstripped the capacity for basic water, sewerage, and sanitation infrastructure and linked to a wider world by high-speed jet, train, or highway travel.

Legionnaires’, however, is the opposite. This isn’t a disease of shanty towns but of luxury hotels and hospitals. It flourishes in conditions of wealth and technology. We have in fact created the ideal environment for this disease by building modernity—specifically, by installing air conditioning and other climate-control technology.

“Their optimal growth range [for Legionella] is from 35 to 45 C (95–113 F),” Dehner writes,

which has important implications for conditions created by modern human settlement components. Provided with single-cell organisms to invade and replicate in, nutrients to feed the growth of these organisms, and water with the right temperature ranges, Legionella are usually present. If the conditions are perfect, ideally with water temperatures in that magic 35 to 45 C (95–113 F) band, Legionella will proliferate; or as the epidemiologists prefer to call it, amplify.

Air conditioning systems create water pools that are at just the right temperature for the bacteria to thrive, and then pump an aerosolized mist of germs into the air. Our options in modern cities for Legionella to make themselves at home are, indeed, legion.

“A dizzying array of modern conveniences have been linked with Legionella outbreaks,” Dehner writes,

including cooling towers, evaporative condensers, steam turbines, showers, hot tubs, humidifiers, decorative fountains, and even a grocery store produce mister. In hospitals, where the patients are particularly vulnerable, infections have been detected through the usual cooling and water and shower mechanisms, but also through aspirators, lavages, and even drinking water.

We’ve inadvertently created a paradise for these bugs, and they seem likely to stick around—not least because our eradication methods are not all that successful.

“The organism is moderately tolerant of chlorine and other disinfectants,” Dehner explains, “so it is likely that some members of the bacteria’s family will survive the standard municipal water treatment processes.”

It also lingers and persists in buildings, sometimes for years. Dehner gives several disturbing examples, including “a Spanish hotel intermittently infected British tourists for over seven years (1973–80) before it was detected. In an outbreak at the Veterans Affairs Hospital in Oakland (Pittsburgh, Pennsylvania), a strain that killed five patients in 2013 was a genetic descendant of a strain first identified in the facility in 1982.”

We don’t even know how many people are sickened by it, as most people presenting at hospitals with pneumonia symptoms are not tested. Legionnaires’: it’s here to stay.

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