Experts are testing two patients suspected of contracting the disease.
An investigation into whether two patients contracted Legionnaires' disease at an Essex hospital has not yet found any other suspected cases.
The inquiry started at Basildon University Hospital when two patients in different wards were suspected of having contracted the illness.
Tests have been carried out and the results are expected on Friday.
A hospital spokeswoman said: "Despite heightened vigilance no further suspected cases have been identified."
Health agency investigating
"It is our normal clinical practice to test patients with a suspicious respiratory infection for legionella, because we are aware of the risk of the presence of the bacteria in the hospital's water system," she said.
"We routinely and regularly treat and check the water system and independent audits are carried out to ensure our processes are rigorous."
She said the last outbreak of the disease at the hospital was in 2007.
The trust and Health Protection Agency are investigating the latest outbreak.
Legionnaires' disease is a potentially fatal 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 its way into artificial water supply systems.
Blood stains found
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 were found on floors, curtains and badly soiled mattresses in the A&E department.
However, on 23 December the watchdog said that hygiene standards were improving and that the trust had taken action to address concerns about infection prevention and control.