The proverbial, reckons Robert Menzies, really has hit the fan. Literally.

Somehow, at Glasgow's flagship NHS hospital, a fungus found in pigeon droppings has found its way in to the bodies of sick patients. One, a child, has died, at least partly as a result.

Mr Menzies spent 40 years designing hospitals. Tiny specks of dust from droppings, he reckons, must have gone through the hospital's air conditioning system, through fans, vents and filters.

That raises two questions, Mr Menzies believes. How did they particles get in to the system? And why did the system itself not stop them getting close to patients?

"The first thing is that it is rare," the 70-year-old architect explains. Birds are always a threat, but infections from fungi in their droppings are unusual. "You are not going to get mass deaths," Mr Menzies said. "But this is unexplained and puzzling."

The Queen Elizabeth Hospital's main new corpus only opened in 2015. Its facade is flush, with no perches. Its roof is flat, however, with a helipad.

So birds could get in, perhaps through a door left open. The health secretary, Jeanne Freeman, has suggested there was a hole, invisible to the human eye, in a plant room on the building's 12th floor. That might be a way in for bugs.

Mr Menzies reckons investigations will begin in that plant room and then follow through the air conditioning to find the bug's pathway to patients.

"There is either a flaw in the design or a flaw in maintenance," he said. There is guidance on where hospitals should install high-efficiency particulate air filters, HEPA filters for short.

These multi-layer fibreglass webs should catch bugs. But no system is perfect, human or machine.

Mr Menzies said it was too soon to speculate about whether the finger of blame could be pointed. "You are not talking about idiots in terms of mechanical and electrical engineers," he said. They know what they are doing. But maybe they have missed something or a set of events has occurred?"

The expert raised the prospect that the mystery may never be solved. What do you do then? "Then you move in to the scenario of patients at risk being re-accommodated somewhere else where the problem does not exist," he said.