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A History of Legionnaire’s Disease
& Statistics

The United States has seen an increase in Legionnaire’s Disease outbreaks throughout the country – but many are still unaware of the history of this disease, as well its causes, symptoms, and potential sources for an outbreak.

With the help of data visualization agency 1Point21 Interactive, we have compiled a comprehensive database of statistics, history, and other notable information regarding Legionnaire’s Disease.

What is Legionnaires’ Disease?

Legionnaires’ Disease is a severe form of pneumonia caused by the Legionella bacteria, a family of microbes that typically resides in sources of fresh water, such as rivers, lakes, and streams. Although they do not typically pose a threat in nature, when they are introduced to manmade sources of water, they can propagate rapidly enough to become harmful when inhaled or ingested.

Symptoms of Legionnaires’ Disease?

If you are infected by Legionella bacteria, your symptoms may develop within 2-10 days of exposure. Symptoms will typically be flu-like at first and quickly ramp up in severity. This can include muscle pain, fever, headache, chills, chest pains, cough, and shortness of breath. In some patients, gastrointestinal symptoms may also develop, such as diarrhea, vomiting, and general nausea.



DAY 10








What is Pontiac Fever?

Some people who are infected with Legionella may develop a milder disease called Pontiac fever. This occurs when a person is infected by Legionella and the bacteria do not enter the lungs. Those with Pontiac Fever only experience the preliminary flu-like symptoms of Legionnaire’s Disease: fever, headache, and muscle aches for approximately two to five days.


death rate for immunocompromised individuals

80%Who do not receive medical treatment

30%Who do receive treatment

How Serious is Legionnaire’s Disease and Pontiac Fever?

Legionnaire’s Disease typically is not deadly for healthy individuals. Those most susceptible to Legionnaire’s Disease are:

  • Greater than 50 years old.
  • Immunocompromised or otherwise have a weak or developing immune system.
  • Those with chronic lung disease or other similar respiratory illnesses.

According to the Centers for Disease Control and Prevention (CDC), approximately 10% of people who contract Legionnaire’s Disease will die from it. However, this number increases sharply when only considering immunocompromised individuals. According to the World Health Organization (WHO), the death rate for immune-suppressed people who are diagnosed with Legionnaire’s Disease is:

  • As high as 80% for those who do not receive medical treatment
  • As high as 30% for those who do receive treatment

Legionnaire’s Disease is best treated with a course of antibiotics, and often requires a hospital stay for continued monitoring – especially in individuals with compromised immune systems. In severe cases, a patient may be admitted to the intensive care unit.

A History of Legionnaire’s Disease in the United States

Although Legionnaire’s Disease has likely existed in some form in the past, the first reported case of the disease was in July 1976 at the Bellevue-Stratford Hotel in Philadelphia. A mysterious outbreak of pneumonia at a convention of the American Legion resulted in 182 infections and 29 deaths.

After a comprehensive investigation, in January 1977 it was determined that the outbreak was due to the growth of a previously unknown strain of bacteria in the hotel’s air conditioning cooling tower. In a nod to the source and origin of the outbreak, the family of bacteria was named Legionella, and the specific species that caused the outbreak was named Legionella pneumophila.

Interestingly, the first reported outbreak of Pontiac fever occurred much earlier in Pontiac, Michigan in 1968. Several workers at the county’s department of health office came down with mild, acute flu-like symptoms to seemingly no explanation. After the Legionnaire’s Disease outbreak in 1977, blood samples from the Pontiac fever outbreak were re-examined, and it was determined to be a match.

How is Legionnaire’s Disease Transmitted?

Legionnaire’s Disease is almost always transmitted through water. It is done so in two different ways:

  • Aerosolized in the form of water vapor
  • Aspirated via a contaminated water source

Legionnaire’s Disease is not contagious – it cannot be transmitted from one infected person to another.

Common Vectors for Legionnaire’s Disease

Legionnaire’s Disease outbreaks most commonly occur due to Legionella contamination and overgrowth in manmade sources of water. This includes:

  • Cooling towers
  • Hot tubs and whirlpools
  • Decorative fountains, such as those found in hotel lobbies and shopping malls
  • Hot water tanks and heaters

Additionally, Legionella outbreaks can occur in sources where water remains stagnant for long periods of time. This includes:

Showerheads and sink faucets

Legionella bacteria may grow on a showerhead or sink faucet that has not been used for an extended period of time. This is most common in places such as hotel rooms and vacation homes.

Old, large plumbing systems

Older plumbing systems that service a large neighborhood or community may harbor Legionella bacteria due to pipes and pathways that have been choked out or otherwise have long been non-operational.

Interestingly, Legionella has also been found in potting soils and other consumer dirt materials – and infection from contaminated soils is possible, but rare.

How Do Legionnaire’s Disease Outbreaks Occur?

Legionnaire’s Disease outbreaks in the United States most commonly occur when multiple people are exposed to water that has had an overgrowth of Legionella bacteria. Legionella thrive and multiply optimally under the following conditions.

Stagnant water

Constant flow can disrupt the propagation of bacteria. Water that is standing in vessels such as cooling towers and fountains can result in significant overgrowth of Legionella.

Warm water

The optimal temperature range for Legionella growth is 77-108 degrees Fahrenheit. Water in cooling towers and other large manmade bodies of water can easily reach this temperature range. In addition, most water that passes through plumbing systems can reach this temperature range. Coldwater that passes through pipes may be warmed up by ambient temperatures, and hot water in some buildings is often not hot enough to completely prevent Legionella growth.

Not enough or no disinfectant

Additives such as chlorine are usually introduced to some bodies of water, such as hot tubs and decorative fountains, to limit and otherwise minimize the growth of Legionella. An inadequate amount of disinfectant can result in rapid overgrowth of Legionella.

Many of the manmade bodies of water that cause Legionnaire’s Disease outbreaks are subject to one or more of these factors. For example, hot tubs are an especially potent source of Legionnaire’s Disease outbreaks due to the interplay between multiple factors. Even if an acceptable amount of disinfectant is added to the water, the inherently high temperatures of a hot tub can more rapidly degrade the disinfectant. If the hot tub is calibrated to decrease the high temperatures, the disinfectant may not be as quickly degraded – but the temperature of the hot tub can be low enough to harbor Legionella growth.

As a result, most Legionnaire’s Disease outbreaks are due to poor and improper maintenance of the aforementioned vessels of water. This can include:

  • Improper cleaning of vessels holding water
  • Failure to store water at an acceptable temperature
  • Failure to update aging pipes in plumbing systems, which can lead to degradation of disinfectant
  • Failure to test the water regularly for microbe growth
  • Inadequate amounts of disinfectant added to water

Ultimately, Legionnaire’s Disease outbreaks are almost always due to negligence.

Legionnaire’s Disease Statistics

Since Legionnaire’s Disease was first discovered in 1977, reported cases of Legionnaire’s Disease have increased exponentially. According to the CDC, the rate of reported cases has grown by 9 times since 2000.

From 2000-2018, the CDC reported 73,462 total cases of Legionnaire’s Disease. 2018 saw a record 9,933 cases of Legionnaire’s Disease. By contrast, 2000 a little over 1,000 cases of the disease.

The most recent Legionnaire’s Disease Surveillance Report released by CDC, for the years 2016-2017, has more statistics regarding Legionnaire’s Disease and its impact on the United States.

  • In both 2016 and 2017, most cases of Legionnaire’s Disease occurred in older individuals. 81% of the reported cases occurred in people 50 years of age or older. Additionally, the highest rate of disease occurred in people 85 years or older.
  • The majority of reported cases were male. In 2016 and 2017, men accounted for 60% and 62% of cases, respectively.
  • In 2016 and 2017, 63% of reported cases were in persons of the white race. However, despite the majority of cases being in white people, African Americans had a higher incidence rate of the disease.
  • In both of the reported years, the vast majority of cases reported were Legionnaire’s Disease cases – in 2016, only 2% were Pontiac fever cases, and in 2017, only 1% were Pontiac fever cases.
  • Hospitalization rates for Legionnaire’s Disease cases in 2016 and 2017 were significant – as high as 96% in 2016 and 97% in 2017.


rates for legionnaire’s disease cases in 2016-2017

97%hospitalization rate in 2017

96%hospitalization rate in 2016

Legionnaire’s Disease Cases by State

In 2016, California had the highest number of reported cases, with 586 people diagnosed with Legionnaire’s Disease. The top 5 states in 2016 were:


New York



In 2017, Ohio has the highest number of Legionnaire’s Disease cases, reporting 601 cases – a 19% increase from the previous year. The top 5 states in 2017 were:


New York



Legionnaire’s Disease Cases in New York City

New York City is a major epicenter for Legionnaire’s Disease cases and outbreaks in the United States. In fact, the number of cases that occur in New York City alone are prevalent enough that the CDC gives the city its own column in its Surveillance Report data – independent of New York State.

According to the CDC, New York City had 268 cases of Legionnaire’s Disease in 2016 and 435 cases in 2017 – representing 58% of the state’s cases in 2016 and 74% of the state’s cases in 2017. Year-over-year, that is a sharp 62% increase in cases in New York City alone.

In 2015, following a cluster of outbreaks in the city that resulted in over 150 cases and 13 deaths, New York City enacted a law that aimed for stricter regulations and cleanliness practices with cooling towers and water towers – thought to be the prevailing vectors causing outbreaks in New York City. These regulations included required disinfection and cleaning mandates for all property owners and building managers in charge of tower maintenance, as well as detailed documentation of every cooling tower in the city to ensure compliance.

However, since those laws have been passed, cases of Legionnaire’s Disease in New York City have not subsided, but rather begun rising more rapidly than before. This may indicate that there are other sources of Legionnaire’s Disease that are not being thoroughly investigated in the city.

Legionnaire’s Disease Cases in the United States vs the World

The rise in Legionnaire’s Disease cases is not isolated to just the United States; some parts of the world are also experiencing a significant increase in cases. This is most notable in Europe, where the number of cases has had a similarly sharp upward trajectory since 2007.

According to the European Centers for Disease Prevention and Control, 2018 saw the highest number of reported Legionnaire’s Disease cases ever, with 11,343 total cases across 30 countries. This is a 23% increase over the number of cases in 2017.

In addition, four countries accounted for nearly 75% of all cases in 2018:

France 2091 cases

Germany 1300 cases

Italy 2874 cases

spain 1492 cases

Of all 30 countries, Slovenia posted the highest rate per capita, reporting 7.7 cases per 100,000. For comparison, the next highest rate is in Italy, with a rate of 4.9 cases per 100,000.

Notable Legionnaire’s Disease Outbreaks in the World

1985 Stafford District Hospital, United Kingdom

In April 1985, 175 patients in Stafford, England were admitted to various hospitals throughout the city for symptoms similar to pneumonia. A medical investigation found the source to be a contaminated air conditioning cooling tower on the roof of the Stafford District Hospital. In all, 28 total patients died due to Legionnaire’s Disease.


This outbreak occurred at the Westfriese Flora, one of the largest indoor flower exhibitions in the world. For ten days, this exhibition included a vendor who had several hot tubs on display for promotional purposes – one of them filled with a seldom-used firehose and heated to 98.6 degrees Fahrenheit, with no chlorine added because it was for display only. These conditions resulted in massive growth of Legionella bacteria – and the resultant water vapor from the heated water enabled a rapid infection of visitors.

Through the next month, over 300 cases were found throughout the Netherlands – with all of them having visited the Westfriese Flora. In all, 318 people were infected and 32 died, and there is a possibility that more may have died.

2000 Melbourne Aquarium, Australia

In April 2000, an outbreak of Legionella in Melbourne resulted in 125 cases and 4 deaths. Upon further investigation, all cases were tied back to a cooling tower at the Melbourne Aquarium – newly opened at that time.

As a result of this outbreak, regulations were passed by the state in 2001 to help curb the spread and infection of Legionella. This includes full, comprehensive documentation of all cases of Legionnaire’s Disease by the state government.

2001 Murcia, Spain

In July 2001, the world’s largest outbreak of Legionnaire’s Disease occurred in Murcia, Spain. From July 7 to July 22, more than 800 suspected cases of Legionnaire’s Disease were treated at the local hospital, and 6 died. An epidemiologic investigation pinpointed the possible source of the infection to a cooling tower at a local city hospital, but the cause has never been definitively determined.

2005 Toronto, Ontario, Canada

In September 2005, residents of Seven Oaks Home for the Aged, a nursing home facility, contracted Legionnaire’s Disease. Within the first week of the outbreak, 21 had died. In total, 127 cases were identified, and the source was traced to the home’s air conditioning cooling towers.

2011 Dayton, Ohio

Contaminated hospital air conditioning resulted in 11 cases of Legionnaire’s Disease and 5 deaths. At the time, this was largest outbreak in Ohio since 1994.

2012 JW Marriott Hotel, Chicago, Illinois

From July to August of 2012, guests at the JW Marriott Hotel in Chicago reported feeling ill after staying at the hotel. 10 total cases of Legionnaire’s Disease were found, resulting in 3 deaths. Further investigations found the source of the outbreak to be the decorative fountain in the lobby of the hotel. Some of the decorative features aerosolized enough water to infect a number of the guests staying there.

2012 Veteran’s Administration Hospital, Pittsburgh, Pennsylvania

In investigation found that, since January 2011, as many as 29 patients at the VA Hospital in Pittsburgh were diagnosed with Legionnaire’s Disease, leading to 6 deaths. According to the VA, only 5 were allegedly contracted at the hospital, with most of the others contracted from unknown sources.

However, an investigation into the outbreak found increasingly negligent acts by the VA hospital that may have contributed to a Legionnaire’s Disease outbreak. Faulty water systems led to the hospital periodically turning off water to certain parts of the hospital – which can promote Legionella growth. In addition, the investigation found that the hospital did not maintain adequate standards for cleanliness. This includes not properly flushing out its water systems, and not properly maintaining the water disinfecting systems in place.

2015 Bronx, New York

One of the largest outbreaks in the New York City’s history occurred through 2015, when four separate clusters of outbreaks throughout the city resulted in over 150 cases of Legionnaire’s Disease and 13 total deaths. 3 of the 4 outbreaks occurred in the Bronx borough, and all were traced to building cooling towers which tested positive for Legionella bacteria.

As a result of this outbreak, legislation was passed in the city, calling for extensive regulations and documentation on cooling towers – thought to be the primary source of Legionnaire’s Disease outbreaks in New York City.

2014-2016 Flint, Michigan

An investigation into McLaren Regional Medical Center in Flint, Michigan found over 87 cases of Legionnaire’s Disease diagnosed at the hospital from 2014 to 2016, resulting in 12 deaths overall. In addition, further investigation found that this number may actually be under-reported, due to the possibility of many cases being misdiagnosed as pneumonia.

As a result of this outbreak, the medical center is currently being sued for $100 million. Although the definitive cause of the outbreak has yet to be identified, experts theorize that the surge in Legionnaire’s Disease may be explained by the poor water quality stemming from the Flint water crisis.

2017 Disneyland, Anaheim, California

In 2017, 22 cases of Legionnaire’s Disease were reported at Disneyland in Anaheim, California, resulting in one death. Amidst intense scrutiny of the theme park’s hygienic practices, an investigation traced the source of the outbreak to cooling towers maintained and managed by the city, not the theme park.

2019 Ohio, New Jersey, Michigan

The Flint water crisis has resulted in a number of Legionnaire’s Disease cases through Ohio, New Jersey, and Michigan. As of June 2019, 32 people have contracted the disease as a result of drinking or being near the contaminated water from Flint, resulting in at least 6 deaths.

2019 Fletcher, North Carolina

At least 141 people were infected and 4 died of Legionnaire’s Disease after attending the North Carolina Mountain State Fair. Similar to the 1999 Netherlands case, a promotional hot tub display promoted the rapid growth of Legionella bacteria – and the resultant water vapor was inhaled by hundreds in attendance.

Why Are Cases of Legionnaire’s Disease Rising?

There are numerous factors as to why Legionnaire’s Disease cases and subsequent outbreaks may be increasing throughout the world.

  • Increased awareness of the disease may contribute to increased testing for the Legionella bacteria.
  • A rising population of people who are more susceptible to the disease, such as the elderly.
  • Increasingly aging plumbing infrastructure through the more urban areas of the world.
  • Climate change, leading to more rapid Legionella growth

It is also important to note that Legionnaire’s Disease is largely considered one of the most underreported diseases in the world. This is due to the fact that Legionnaire’s Disease does not necessarily present symptoms unique to itself – it is commonly misdiagnosed as the flu or standard pneumonia. In fact, according to the National Academies of Sciences, Engineering, and Medicine, there may be as many as 70,000 cases of Legionnaire’s Diseases every year in just the United States alone.

The most common reason for outbreaks is inadequate maintenance of any large manmade bodies of standing water.

Who is Liable for a Legionnaire’s Disease Outbreak?

The most common reason for Legionnaire’s Disease outbreaks is improper or inadequate maintenance of any large manmade bodies of standing water. For example, much of the outbreaks in New York City occur due to improper monitoring and maintenance of cooling towers on buildings.

As a result, when determining liability, it is almost always the person or entity responsible for maintaining any relevant bodies of water that assumes full liability for any resultant Legionnaire’s Disease outbreak.

While determining the liable party may be fairly straightforward in most outbreaks, some situations require further investigation before determining definitively who was responsible for the outbreak. Regardless of the difficulty in determining a liable party, it is highly recommended to consult a Legionnaire’s Disease attorney who has prior experience with Legionnaire’s Disease outbreak cases before proceeding with a claim. They may be able to assist you in your efforts to pursue compensation for your damages, first by figuring out who is responsible for your damages.