Experts are testing two patients suspected of contracting the disease.
A possible outbreak of legionnaires' disease has been reported at a hospital in Essex.
An investigation has been started at Basildon University Hospital after two patients were suspected of having contracted the illness.
Tests are now being conducted to discover the source of the suspected outbreak, a hospital spokeswoman said.
Swabs from both the patients, who were staying in different parts of the hospital, have been sent for tests.
The spokeswoman said: "We are investigating two suspected cases of legionnaires' disease.
"We do lots of testing for legionella in patients who have suspicious respiratory infections.
"We have had problems with the disease before so we are acutely aware of the risk of the bacteria.
"We have particularly high levels of control and are constantly performing checks and taking preventative measures."
She said the last outbreak of the disease at the hospital was in 2007.
"It is accepted by experts that it is practically impossible to eradicate the legionella bacteria completely on an ongoing basis from large and complex water systems," she added.
Legionnaires' disease is a potentially infection that is caused by the bacteria legionella.
The bacteria is commonly found in sources of water such as rivers and lakes but can sometimes find their way into artificial water supply systems.
It is estimated 10% of people who contract legionnaires' disease will die from complications arising from infection.
In November, inspectors from the Care Quality Commission criticised Basildon and Thurrock University Hospitals NHS Foundation Trust.
Blood stains had been found on floors and curtains and badly soiled mattresses in the A&E department with stains soaked through were discovered.
However, on 23 December the watchdog said that hygiene standards were improving and that the trust has taken action to address concerns about infection prevention and control.