A fungal infection caused by pigeon droppings has been detected in two patients at the Queen Elizabeth University Hospital in Glasgow.
A non-public room, thought to contain machinery, was identified as a likely source and the droppings removed.
NHS Greater Glasgow and Clyde (NHSGGC) said the organism is a Cryptococcus species, which is harmless to the vast majority of people.
BBC Scotland understands the patients are not seriously ill.
An investigation is under way and control measures are in place.
NHSGGC confirmed a small number of vulnerable paediatric and adult patients are receiving medication to protect them against the airborne infection.
Portable HEPA air filter units have been installed in specific areas as an additional precaution.
Teresa Inkster, lead consultant for infection control, said: "Cryptococcus lives in the environment throughout the world. It rarely causes infection in humans.
"People can become infected with it after breathing in the microscopic fungi, although most people who are exposed to it never get sick from it.
"There have been no further cases since the control measures were put in place."
Ms Inkster said experts are continuing to monitor the air quality.
She added: "It remains our priority to ensure a safe environment for patients and staff."
'Very unusual'
Prof Hugh Pennington, of Aberdeen University, said he was surprised to learn of the infection.
The epidemiologist said: "It is very unusual in the UK.
"It is quite common in other parts of the world, particularly in tropical parts and in the US and in countries like that, where they have more problems with this particular kind of fungus."
Prof Pennington said people with weak immune systems are most at risk.
He added: "When it gets into the blood stream a lot of people have fairly straightforward infections and it settles in the lungs but the big problem with this is that it can cause meningitis and, as we know, meningitis can be a very serious infection."
Prof Pennington said anti-fungal drugs are used to treat the infection but warned it can be fatal if it is not diagnosed.
Airborne infection
The expert said a key priority would have been stopping the airborne infection from entering the hospital's ventilation system.
He added: "Obviously they have stopped the pigeons getting into the machine room.
"It surprises me slightly that there was any there in the first place."
During the investigation, a separate issue arose with the sealant in some of the shower rooms.
NHSGGC said repairs are underway and our maintenance team are working to remedy this issue as quickly as possible with the minimum disruption.
As a further precaution, a specific group of patients are being moved within the hospital due to their clinical diagnosis and ongoing treatment.