Following the blast of Arctic weather that hit the UK last week, it may have been the very last ailment expected to put the average commuter in hospital.
But for travellers in the south-east, the sudden unseasonal upswing in temperatures on Tuesday was enough to shut down a major rail hub when four passengers were simultaneously taken unwell after overheating in their winter garments.
As London went from biting Scandinavian snow drifts that cancelled hundreds of flights at Heathrow to balmier weather blowing in from the Azores in less a week, the London ambulance service (LAS) confirmed that three rush-hour commuters were taken to hospital with what initially appeared to be a mystery illness.
LAS tweeted that it was treating people at London Bridge station following "an incident".
LAS confirmed that after a call at 8.39am it sent its hazardous area response team to the scene while witnesses reported that more than a dozen police cars and fire engines were in attendance.
The station, which serves tens of millions of underground and rail passengers each year, was closed and cordoned off as authorities enacted the Health Protection Agency's "step 1-2-3" guidance, which states that if three or more people are taken unwell at the same time and the cause is not known, those who are ill should not be approached and responders should "withdraw and send for specialist help".
Lucinda Stump, 26, whose workplace overlooks London Bridge, said that scores of firefighters were in attendance and she believed they were dealing with a major incident.
The news began trending on social media, with stranded passengers and the public speculating that a chemical leak or even a terrorist attack could be the ultimate cause of the undiagnosed illness.
In the end the most probable cause turned out to be less deadly but just as unexpected: overheating in the middle of January.
After a sudden 10C rise in temperatures in just under a week, the passengers admitted to hospital encased in one too many layers of clothing were overcome by the traditionally unpredictable British weather.
Helen Chivers from the Met Office said it was no surprise that people were getting caught out.
Whereas last Tuesday's maximum daytime temperature was just 3C, a week later the morning temperature in the capital was 11C.
"To have a temperature very early in the day as high as 11C, well, I would think is pretty unusual in January," she said.
Although Tuesday's maximum temperature of 14C in the capital was some way off the 27 January 2003 record of 17.6C, Chivers said the "rise in temperatures has been pretty marked".
She added: "Where the previous system was coming from Scandinavia, what's happened over the last 24hrs is that the air that we've got at the moment has got relatively tropical origins.
"It's come to us from the Azores and that's why there's such a big difference in the temperatures."