Some guests to Disneyland this summer left with more than just a belly full of cotton candy, Mickey Mouse ears, and a phone full of selfies. On November 11, health officials in Orange County, California reported that Legionnaires’ disease sickened 12 people in and around the theme park in September, including 9 Disneyland patrons. On Wednesday, the Orange County Register reported that the number of total cases has risen to 15, of which 11 recently visited Disneyland. Following an investigation, health officials identified two cooling towers in the theme park that tested positive for elevated levels of the Legionella bacteria responsible for the deadly disease. Disneyland has since taken the two cooling towers offline, but other potential sources are still being investigated. Orange County health officials state that “there is no known ongoing risk” to Disneyland guests.

The national incidence of Legionnaires’ disease rose fourfold from 2000 to 2014, and the Orange County Health Department stated that the number of Legionnaires’ cases in the County and in southern California, specifically, has jumped in recent years. This upward trend is not limited to the US, however, as the European CDC reported a threefold increase in Europe from 1995 to 2013. Of note, Lisbon, Portugal is also currently battling a Legionnaire's outbreak that has resulted in 46 cases and 5 deaths since late October. Below, we explore some of the seasonal, climatic, and geographic trends associated with higher risk of Legionnaires’ disease, and we consider possible explanations for the rising global incidence.

Background on Legionnaires’ Disease

Discovered in 1976, Legionnaires’ disease, a severe form of legionellosis, is a serious respiratory illness caused by bacteria of the genus Legionella. The bacteria live primarily in fresh water and are spread to the lungs via aerosolized droplets. The bacteria are only dangerous at high concentrations, but water supplies that are not regularly cleaned and disinfected may become contaminated with dangerous levels of Legionella. According to the US CDC, outbreaks of Legionnaires’ disease “are often associated with large or complex water systems, like those found in hospitals, hotels, and cruise ships.” The most likely sources of infection include contaminated water used for showering, cooling towers, hot tubs, and decorative fountains.

According to the WHO, symptoms range in severity from mild influenza-like symptoms to severe pneumonia and include fever, headache, lethargy, and muscle pain. Individuals with weak immune systems, the elderly, smokers, and those with chronic illnesses are at higher risk. In fact, the case fatality for hospital-acquired cases is 2.5 times that of community-acquired infections. Therapy typically includes administration of first-line antibiotics, and studies show that overall efficacy is good for many classes of antimicrobial agents.

Timely reporting of suspected Legionnaires’ cases enables health officials to identify and stop potential outbreaks. This typically involves linking suspected cases to a common source, which is then disinfected or shut down, as occurred in the first, now famous, outbreak of Legionnaires’ in Philadelphia in 1976. Typically, fewer than 5% of persons exposed to a contaminated water source will become infected, which helps explain why only a small percentage of Disneyland guests contracted the illness. Conversely, the US CDC notes that Legionnaires’ disease is often underdiagnosed, leading to underestimates in infection rates. Person-to-person transmission is rare but has been documented.

Summer in the City

Studies of US weather data indicate that the incidence of community-acquired Legionnaires’ disease “varies considerably by season and local weather patterns.” Generally, warm (60°-80°F) and humid (>80.0%) weather and rainfall are associated with increased risk of legionellosis; however, the exact mechanism by which these factors increase risk remain unclear. It is, perhaps, no surprise that the East Coast has the highest percentage of “bacterial pneumonia discharges for which an [Legionnaires' disease] diagnosis had been given,” considering prevalent humidity and warm temperatures outside of the winter months. Legionella bacteria thrive in “warm, wet environments” which support its survival and, potentially, its ability to aerosolize. Incidence of Legionnaires’ disease also tends to increase during the summer months. In fact, 62% of reported cases in the US occurr between the months of June and October, when air-conditioning units and cooling towers tend to be in operation. In contrast, community-associated Legionnaires’ is “comparatively rare” in overly hot, dry climates or cooler temperatures such as the environments found in the Southwest region and the Rocky Mountains.

In 2005, Legionnaires’ incidence was 10 times higher in New York state than in California, which reports, on average, about 250 cases of legionellosis per year. The incidence was only 2.5 times higher in 2016 (with an incidence rate of 3.70 per 100,000 persons for New York and 1.49 for California; based on US Census Bureau population estimates). California’s comparatively lower incidence may, in part, be explained by its climate which, compared to New York, may generally be less conducive to Legionella growth. Much of California tends to be drier and less humid than New York, especially during warmer months, and the wetter season tends to coincide with the winter, during which temperatures are likely below the ideal range for Legionella. For example, average September temperatures in Anaheim, CA range from a low of 63°F to a high of 87°F, which falls squarely in the temperature range correlated with increased Legionnaires’ cases; however, average precipitation for September is typically less than 0.25 inches and humidity rarely rises above 60%, which is not particularly conducive to Legionella bacterial growth. This summer’s weather appears consistent with these trends, with no unusual weather events, such as El Niño, which can cause increased coastal rainfall and flooding.

Investigating the Rise in Cases

Due to the myriad of factors that affect the incidence of Legionnaires’ disease, it is difficult to attribute the rise in incidence to one or more specific factors, and further investigation of this upward trend is required to fully characterize the issue. Possible explanations include increased susceptibility among the general population (eg, due to increasingly aging populations and comorbidities); deterioration in water and plumbing infrastructure; and increased awareness, testing, and reporting by clinicians. It is also possible that climate change is playing a role through an increase in total rainfall levels, rising temperatures (and the associated reliance on air-conditioning systems), or other pathways. In fact, environment-acquired infections have been recorded in recent years in highly unexpected locations, such as a November 2012 outbreak in cool, dry western Canada.

The growing incidence of Legionnaires’ disease suggests that health systems and municipalities will need to step up surveillance, reporting, and preventive measures. Following a 2015 outbreak in the Bronx during which 138 people were infected, New York City passed comprehensive legislation to regulate and inspect cooling towers. According to officials from the New York City Department of Health and Mental Hygiene, there was an “urgent need to shift from relying on the alarm bell of human disease to primary prevention strategies designed to limit Legionella colonization and dispersal from human-made aerosol-generating devices.” Other cities at risk for Legionnaires’ disease may want to consider adopting proactive legislation and policies focused on the regular maintenance and cleaning of water and air conditioning units in order to prevent or mitigate the risk of future Legionnaires’ outbreaks.


Outbreak Observatory aims to collect information on challenges and solutions associated with outbreak response and share it broadly in near-real time to allow others to learn from these experiences in order to improve global outbreak response capabilities.

Photo: Legionella pneumophilia bacteria
Photo courtesy of CDC/Janice Haney Carr