Plymouth City Council is investigating an isolated case of Legionnaire's Disease in the city.
One person is being treated at Derriford hospital for the effects of the disease, which is contracted by inhaling water infected by the bacterium.
Public Health England is investigating with the Council.
Launceston General Hospital will close temporarily so the water system can be disinfected. It's expected to take four weeks and will cost more than £100,000.
Launceston General Hospital in Cornwall has closed to the public from today after traces of legionella bacteria were found in its water system.
The bug, which causes the potentially fatal Legionnaires Disease, was found in two locations. The community hospital will now close for 4 weeks to allow a complete disinfection.
Peninsula Community Health, which runs the hospital said there was "no reason to suspect that public, patients or staff have been exposed to a specific risk of infection."