Medical experts say the latest strain of enterovirus D68, a virus that has caused severe respiratory problems in children across North America in recent months, is behaving in a new way -- and they want to know why.

Health officials began to notice a problem in late August, when children first thought to be battling a mild cold began developing severe breathing problems.

In Montreal, eight-year-old Renato Girolami started with a mild cold that quickly worsened. Eventually he was admitted to hospital, where tests showed that the young boy was indeed infected with the virus.

“It was hard to breathe,” Renato said. “When I was walked, it was hard to walk.”

His mother Paula Girolami said it was a frightening experience.

“He’s been healthy all his life and it just happened so suddenly,” Girolami told CTV News.

Renato’s health gradually improved, and after six nights, he was able to come off oxygen and return home.

Some children have not been able to overcome the virus’ effects. EV-D68 is now linked to a handful of deaths in the U.S., including a 21-month-old girl who died Friday in Michigan.

“It is a virus that’s been circulating more than it ever has before,” said Dr. Jeff Pernica, infectious diseases specialist at McMaster Children’s Hospital in Hamilton, Ont.

“It may be slightly more severe than other respiratory viruses.”

Scientists are taking to the lab to figure out how the virus’ latest strain is different from previous strains, and why it is causing respiratory problems in children.

Physicians say they don’t know if the old virus has changed for the worse, or if children today are susceptible to its effects. They also don’t know whether EV-D68 is linked to a mysterious spike in paralysis cases in children.

“I really do think it is a marked increase incidence over what we expect to have happen,” Dr. Joan Robinson, an Alberta-based pediatric infectious disease expert, told CTV News.

EV-D68 was implicated in a cluster of polio-like cases in California two years ago. It affected 3-year-old Lucian, who lost function in one leg. His mother, Erin Olivera, says she is distressed that new cases are emerging.

“Each day, more and more are showing up,” she said. “It’s very heart-breaking.”

Scientists wonder if the virus is causing the body’s immune system to do this type of damage.

More questions abound regarding the geography of the virus: there are signs EV-D68 is on the wane in some regions, but on the rise in others.

“Within the next few weeks, a lot of the studies that we will have ordered to try and pin down the enterovirus will be back,” Pernica said.

In the meantime, doctors say parents should encourage their children to wash their hands regularly. And if their children show signs of breathing problems, they should seek medical care.

With a report by CTV News Medical Specialist Avis Favaro and producer Elizabeth St. Philip