An unexpected characteristic has emerged among many swine flu victims who become severely ill: They are fat.

Doctors tracking the pandemic say they see a pattern in hospital reports from Glasgow to Melbourne and from Santiago to New York. People infected with the bug who have a body mass index greater than 40, deemed morbidly obese, suffer respiratory complications that are harder to treat and can be fatal.

With the new virus on a collision course with the obesity epidemic, the World Health Organization says it’s gathering statistics to confirm and understand this development. Drugmaker Roche Holding AG is combing through studies to determine whether heavier people should get bigger doses of its Tamiflu antiviral.

“Morbid obesity is one of the most common findings turning up in severely ill patients,” said Nikki Shindo, who is leading the investigation of swine flu patients at the WHO in Geneva. “It’s a huge problem.”

It couldn’t be that fat people are more prone to health and associated problems, could it, rather than swine flu itself? Or am I not taking swine flu seriously enough?

Scientists don’t yet know whether extremely overweight people get sicker because of associated conditions like heart disease and asthma, or whether the excess fat itself makes them more vulnerable. Both may be to blame.

Fat cells secrete chemicals that cause chronic, low-level inflammation that can hamper the body’s immune response and narrow the airways, says Tim Armstrong, a doctor working in the WHO’s chronic diseases department in Geneva.