A meat-boning factory is being investigated after two of its workers contracted Legionnaires' disease.
Water systems at the plant in Walton Summit near Bamber Bridge, Preston and nearby cooling tests are being tested for Legionella bacteria.
The Health Protection Agency (HPA) said the cause of the outbreak has not yet been established.
One of the workers has made a good recovery but the other is still poorly, the HPA said in a statement.
Infection sources
An incident team, with representatives from the HPA, the Health and Safety Executive, Central Lancashire PCT and South Ribble Borough Council, has been convened to oversee the investigation.
Dr Sohail Ashraf, Consultant in Communicable Disease Control with the HPA's Cumbria and Lancashire Health Protection Unit, said: "It's important to stress that we do not know the source of this infection at this stage, but as both patients worked in the same factory, we are checking water systems there as a starting point to the investigation.
"We are also looking at other potential sources of infection in the locality."
Legionnaires' disease is a form of pneumonia that older people and people with other underlying health conditions can be particularly vulnerable to.
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