It started with weakness in her left leg and pain in her lower back. Then, there were rashes "that felt like little sewing needles" stabbing her.

"It was difficult to walk for a couple of days -- the nerve pain was pretty bad," said Lorena Rosati, who works for CTV News.

"It was in my back and then travelled down my leg as well, so sitting all day was extremely uncomfortable and I had some trouble sleeping at night."

When Rosati, 36, visited her doctor, she was shocked to learn that she had shingles.

"I actually started laughing at first because I couldn't believe that it would be shingles," she said. "You don't usually hear about a younger person having shingles, so that was quite surprising."

While Rosati is now better, she is one of as many as 130,000 Canadians who develop shingles each year in Canada. Both Canadian researchers and the U.S. Centers for Disease Control have noticed that the number of shingles cases has been increasing steadily among adults for years, but no one is quite sure why.

Anyone who has ever had chickenpox is at risk of shingles. That's because even after chickenpox clears up, the virus that causes it (the varicella-zoster virus) doesn't leave the body but instead goes dormant until the body's natural immunity wears off. Then, the virus can reactivate as shingles, explains infectious diseases expert Dr. Fred Aoki of the University of Manitoba.

"The rash wakes up in one or more of these nerves, crawls out along that nerve, and when it reaches the skin, it causes clusters of blisters or a red patch," he said

The rash is usually in a stripe on the trunk of the body, but it can also show in patches elsewhere on the body, and rarely in the eye.

Why the virus reactivates in some and not others is a mystery. It's clear the risk increases with age, but younger people can develop the illness, sometimes because of conditions that weaken the immune system, or intense stress.

Because shingles is not thought of as a disease that younger people get, they often don't recognize its symptoms. They might think they're coming down with the flu or think they strained a chest muscle, when in fact they are in the early stages of the illness.

Getting a diagnosis early is important, says infectious diseases expert Dr. Lynora Saxinger from the University of Alberta Hospital.

"If you start treatment for it, you can reduce the severity and reduce the risk of going on to a pain syndrome that can sometimes be prolonged," she told CTV News. "You can actually improve your chances of healing well by getting therapy early."

The pain syndrome Dr. Saxinger is referring to is called postherpetic neuralgia, or PHN. About 10 to 15 per cent of people who get shingles develop the complication, which causes severe pain in the areas where the shingles rash occurred that can extend for weeks or months. In some cases, the condition is permament.

The older a shingles sufferer is, the more likely they are to develop PHN and the more painful it can be, says Dr. Allison McGeer, an infectious diseases expert at Mount Sinai Hospital in Toronto. That’s why people over age 50 are urged to get one of the two shingles vaccines now on the market. (Ontario covers the cost of shingles vaccination, although only for seniors between the ages of 65 and 70.)

“The value of the shingles vaccine is not in preventing shingles; it’s in preventing postherpetic neuralgia,” she told CTVNews.ca.

For younger adults in their 30’s, 40’s, Dr. Saxinger says it’s important they are aware that they too are at risk of shingles. She says recognizing the symptoms of shingles early is key because a doctor can prescribe antiviral medications that can shorten the illness as well as reduce the risk for PHN.

But another key reason to see a doctor, she says, is to investigate whether there is an underlying health condition that triggered the shingles in the first place.

"Shingles in a younger person could be a sign of another underlying health condition. So if you get shingles, it might be worth having a check-up to make sure you don't have another health condition that hasn't been recognized," she said.

One other theory that might explain the rising incidence of shingles is that the chickenpox vaccine that most children now receive has reduced the amount of "wild" varicella virus circulating in the air. Some studies have shown that being exposed regularly to the chickenpox virus helps keep up our immunity.

But the CDC says this proposed explanation seems "unlikely" since more recent studies have found that shingles rates were on the rise before the chickenpox vaccine came into wide use in the late 2000s in Canada and the U.S., and that rate hasn't gone up sharply since. A large study on shingles incidence in Alberta made similar findings.

Dr. McGeer says it seems that the long-term effects of not being regularly exposed to the chickenpox virus are still unclear.

"What we don't have is a good sense is how much impact a lack of re-exposure has at a population level," Dr. McGeer told CTVNews.ca.

With a report from CTV’s medical specialist Avis Favaro and producer Elizabeth St. Philip