For Joey Pietraroia, one of nature's nasty surprises began with a good joke. Nine years ago, when Pietraroia was 31, he sat down to Christmas dinner with friends.

"We were at the table and someone made a funny comment," he recalls on the line from Victoria, where he is conductor in residence with the Victoria Symphony and Pacific Opera Victoria. "I was hysterical, and as I laughed I started coughing. I could feel my chest getting tighter and tighter, and I was having trouble breathing. I thought I was just coming down with something and that I'd be fine when I got home. But an hour later I was still coughing, and it didn't get any better. So I was up all night, with my chest tightening up. I'd never experienced anything like it before."

Thus began weeks of similar attacks, most often in the early hours of the morning, when Pietraroia would wake up hacking and "feeling like I wanted to scratch the insides of my lungs". After he'd made a string of visits to specialists, including a couple of trips to emergency when the symptoms became severe, doctors determined that he was probably suffering an asthmalike allergic reaction to cat dander. This came as a shock. Although Pietraroia owned no pets, he'd been around them all his life without any inkling of trouble.

Even so, the sudden appearance of an allergy after decades of symptom-free health is not uncommon. In fact, according to Amin Kanani, clinical instructor in the allergy and clinical immunology division at UBC's department of medicine, the phenomenon reflects a rapidly growing trend.

"Allergies in general are increasing, compared with 30 or 40 or 50 years ago," Kanani notes when reached at his office. "It's not unusual to see allergies starting in people in their 20s, 30s, or 40s. Generally, we see it more in children, but it's not completely unusual to see it later in life."

Research, he says, has revealed that genetic predisposition plays a big role in settling who develops allergies and who doesn't. But allergy rates, including those for allergies that appear suddenly in adulthood, are climbing at a clip in Canada and elsewhere in the western world. One example gives an idea of how fast: the Mount Sinai School of Medicine in New York tracked the rise of peanut allergies in the U.S. between 1997 and 2002 and concluded that prevalence had doubled in that span. Explanations as to why remain elusive, although evidence is beginning to point in certain directions.

"One of the main theories is the hygiene hypothesis, and it just means we're a bit too clean here in western society," Kanani says. "As we're growing up, we're not exposed to as many viruses and bacteria and other infections as, say, someone who's growing up in a developing country. And so our immune system is not constantly active, fighting off infections. It's sitting a bit idle, and therefore we start to produce antibodies against things like pollen and cat dander. It has nothing to do, so it starts to develop these allergic reactions. That's just the main theory out there."

This, however, does little to explain why an initial allergic reaction appears when it does, especially in the case of someone who, like Pietraroia, has already lived well into adulthood with no drastic changes in his environment. Here, Kanani says, the search for answers gets truly murky.

"We can sometimes be exposed to something for a while-for five or 10 or 15 years-and then develop an allergy to it," he observes. "We know that certain people are born with a predisposition, but why an individual develops it after 10 years of cat exposure as opposed to another individual who may develop it after two-that we cannot completely explain. We don't know why a specific switch occurs at a given time."

On top of this, new forms of adult-onset allergy are emerging. Among them is oral-allergy syndrome, a condition that has arisen over the last 20 years in which a person who has suffered in their younger years from a run-of-the-mill pollen allergy suddenly finds, in their 30s, 40s, or 50s, that they have allergic reactions to tree fruits (most commonly apples, nectarines, and plums) and nuts (such as almonds, hazelnuts, and walnuts), as well as certain vegetables, like carrots and celery. It's all to do with the body mistaking proteins in these items for similar proteins in tree and grass pollen, Kanani says, then reacting with hay fever-like runny nose, congestion, and watery eyes.

According to the Toronto-based Allergy/Asthma Information Association (www.aaia.ca/), other symptoms include itching, tingling, or swelling in the lips, tongue, and throat, as well as rashes or swelling where juice from the raw fruits or vegetables touches the skin. If you have any of these reactions, you should stop eating the food in question immediately to avoid any possibility of provoking severe symptoms, the Web site notes. You should also see your doctor about treatments such as antihistamines.

The upward trend in overall allergy rates-including those that seem to fall from the sky onto unsuspecting adults-is concerning, Kanani notes, particularly with the higher rates of associated asthma.

"Unfortunately," he says, "we don't have clear answers yet. Right now, there's promising things in terms of treatment for these conditions. But I think it's still a while to go before we've got good, clear treatments that can help prevent the development of allergies-because part of it is genetic, and that part is difficult to change."