The hospital is the probable source, the chief executive said
Two patients at Basildon University Hospital in Essex have Legionnaires' disease, it has been confirmed.
Tests were carried out at the weekend after two people were suspected of having contracted the illness.
Chief Executive Alan Whittle said the hospital was the probable source based on tests of water samples.
He said both patients had responded to antibiotics, although one patient was still in a critical condition. No more suspected cases have been identified.
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 their way into artificial water supply systems.
Mr Whittle said: "We routinely and regularly treat and check the water system and independent audits are carried out to ensure our processes are rigorous.
"Experts agree that the legionella bacteria is a common risk in large buildings with an extensive plumbing system.
"We accepted some time ago the advice of experts that we will never be able to completely eradicate the bacteria, but we have worked hard to minimise the risk."