A recent outbreak of Legionnaires' disease in Milan, Italy, has killed at least four people and hospitalised several dozen more, according to The Metro. The health authorities say that at least 53 cases have been confirmed in two regions inside the Milan metropolitan area of a severe lung infection.

In response, local authorities are taking urgent measures to control the situation, including deploying chemicals to disinfect the city's water supply. However, despite extensive testing of water supply systems in private residences and cooling towers, the exact source of the outbreak remains unidentified, according to the news report.

The continuous investigation of finding the source of infection has left the city in great suspense, and sensitivity has developed regarding what efforts should be present to prevent further cases.

What is Legionnaires' disease?

According to a Washington Post report, Legionnaires' is a serious form of pneumonia caused by the bacteria Legionella. The pathogen can also cause Pontiac fever, a flu-like illness considered to be a less serious disease.

The bacteria was first discovered decades ago when a mysterious illness sickened dozens attending a 1976 convention of the American Legion in Philadelphia. In that outbreak, 221 people were infected, and 34 died.

The disease initially alarmed health officials, who feared the sickness might spread beyond the conferencegoers. Ultimately, scientists found the bacteria bred in the water of a cooling tower of the air-conditioning system at the Bellevue-Stratford Hotel, where the conference was being held.

How does Legionnaires' disease spread?

Legionella grow in biofilms lining water pipes, pipes that lead to showers, drinking faucets, and HVAC systems for cooling, said Panagis Galiatsatos, an associate professor of medicine in pulmonary and critical care medicine at the Sidney Kimmel Comprehensive Cancer Centre at Johns Hopkins University.

Legionella can even grow in the tanks that hold windscreen wiper fluid in a car, according to the Centres for Disease Control and Prevention.

People can become infected by breathing in mist or swallowing water into the lungs that's contaminated with the bacteria. "Less commonly, people can get sick by aspiration of drinking water containing Legionella," a CDC spokeswoman said Tuesday.