A patient may have become infected with Legionnaires' disease while at a hospital's dialysis unit.
Legionella bacteria was found in a patient at Gloucestershire Royal Hospital in October.
Gloucestershire Hospitals NHS Foundation Trust said the patient "may have acquired" it at the unit.
The hospital said no other patients were affected and "enhanced control measures" removed the risk of it spreading.
What is legionnaires' disease?
- It is a potentially fatal lung infection caused by Legionella bacteria.
- It is not contagious and cannot be spread directly from person to person.
- It is caught by breathing in small droplets of contaminated water.
- Initial symptoms include a high fever and muscle pain.
- It is treated by intravenous antibiotics.
- Legionella bacteria are commonly found in sources of water, such as rivers and lakes, which sometimes find their way into artificial water supply systems.
- NHS advice is that large buildings such as hotels, hospitals and office blocks are more vulnerable to Legionella contamination because they have larger, more complex water supply systems and the bacteria can quickly spread.
- Legionnaires' disease is three times more common in men than women and mostly affects people aged over 50.
- An estimated 10% of people who contract the disease will die from complications arising from infection.
A trust spokesman said: "We recognise that this must be a distressing time for our patient and we have been keeping the patient fully informed about this isolated event.
"We are taking further precautions by increasing our water testing to minimise further the chance of this occurring again."
A joint review by Public Health England and the hospital will consider whether procedures need to be tightened to prevent it from happening again.