The rare and potentially lethal form of pneumonia was diagnosed at Cheltenham General Hospital, run by Gloucestershire Hospitals NHS Foundation Trust.
The Health Protection Agency (HPA) and trust are working together to find a source as they do not know whether it was contracted on the premises.
Environmental health officers from Cheltenham Borough Council are also helping with investigations.
Measures have been taken to isolate the patient, who is still in hospital. The hospital water system has been chlorinated and heat-treated. The Hazelton ward was closed to new admissions but is now open as normal and bottled water has been supplied to other patients on the ward to drink and wash with.
The disease is caused by a bacterium and is contracted from breathing in tiny water droplets from an infected source. It is most commonly spread through contaminated air conditioning systems, but can also be contracted from inhaling mists from baths and showers. There are roughly 500 cases in England each year and most of these are individual cases.
Dr Isabel Oliver, South-West consultant epidemiologist from the HPA, said: ‘People with Legionnaires’ disease may develop pneumonia so it’s important to investigate these cases thoroughly. However, we would like to reassure the public that the disease cannot be spread from person to person.’