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Chocolate has always been a food that fires up the senses -- think velvety, think creamy, think rich, smooth, romantic or sinful. But who thinks salmonella?

The discovery of salmonella in an ingredient at a Hershey's factory in Ontario came as bittersweet news to chocolate lovers. After all, a bacterium most people associate with undercooked chicken was being linked to one of our greatest pleasure foods.

While you're still far more likely to get salmonella from a hamburger than an Oh Henry! bar, experts say chocolate can be particularly vulnerable to contamination.

The Hershey's case, for example, which prompted a national recall on the weekend, marks the third time in five years the nasty bug has been linked to the world's best-loved confection.

"This isn't surprising. Outbreaks of salmonella related to chocolate occur on a fairly frequent basis," said Mansel Griffiths, director of the Canadian Institute for Food Safety Research in Guelph, Ont. "Chocolate is . . . a good environment for salmonella to survive."

One culprit is the very thing that makes chocolate so good: its high fat content, which may help protect the bug from stomach acids that could otherwise help to destroy it.

"Chocolate is high in sugar and fat, so it helps the organism survive its transition through the stomach," said Prof. Griffiths, who teaches in the food sciences department at the University of Guelph.

In fact, in chocolate, salmonella can survive temperatures of 125 degrees for 20 minutes -- conditions that would otherwise kill it, said Allison McGeer, a microbiologist at Mount Sinai Hospital in Toronto.

Prof. Griffiths estimates salmonella cases related to chocolate occur somewhere in the world about once a year.

The cases tend to carry with them an extra dose of consternation because of chocolate's status as a comfort food.

"It's a treat, something that we take pleasure in eating," he said. "So it's psychological. The things we like, we don't expect to make us ill."

Dr. McGeer predicts the world is likely to hear about a lot more cases of tainted chocolate and other food contamination in the future due to better international surveillance.

"You're going to hear about it because we are getting better at detecting it, she said. She said sharing lab results on the Internet has become necessary because food is distributed so widely.

"Look at what happened with spinach," she said, referring to recent cases of E. coli. "You can have one farm, one day, have one thing happen with one lot of spinach, and it hits 50 states and four provinces."

Chocolate, she said, can be tougher to track because the cases of illness "might propagate slowly" as the shelf product is shipped far and wide. But, she noted, because salmonella can survive well in chocolate, people need not consume large amounts of the bacteria to become sick.

This summer, Cadbury recalled a million chocolate bars in Britain and Ireland after salmonella was found at one of its factories and health officials noted a spike in salmonella cases, especially among children.

During the winter of 2001, an international outbreak of salmonella that sickened more than 300 people was traced back to German chocolate. A 2005 report from epidemiologists in Berlin found that tainted German chocolate had also made its way to Canada.

Living naturally in the guts of animals -- poultry and pigs in particular -- the bug is far more commonly linked to things like undercooked meat or raw eggs. But salmonella can, through improper food handling, or unpasteurized milk, or perhaps even tainted nuts, find their way into chocolate.

"The conditions exist to allow the growth of salmonella in chocolate," said Marie-Andrée Lefebvre, a spokeswoman for the Canadian Food Inspection Agency. "It has no animal products, so it has to come from one of its ingredients."

The suspected source in this summer's Cadbury's recall, for example, involved a leaky water pipe contaminating a batter of milk, cocoa and sugar.

But tracking down the exact source can be difficult.

Once salmonella gets into the environment, it can survive up to 200 days, said Keith Warriner, a professor of food microbiology at the University of Guelph.

Hershey's has so far refused to reveal the product that tested positive for salmonella at its factory, prompting a shutdown last week. It caused a recall of more than two dozen products over the weekend, including Reese's Peanut Butter Cups and Lowney Cherry Blossom.

A company spokeswoman yesterday said the culprit is "externally sourced" and was not a major component of chocolate, such as milk or cocoa. So far, the company has received no confirmed reports of illness.

Despite this weekend's recall, chocolate is still a minor player when it comes to salmonella poisoning over all. Of the 6,000 to 12,000 cases of salmonella reported in Canada each year, about 50 per cent are linked to undercooked eggs or meat, Prof. Warriner said. Fresh produce accounts for another 19 per cent.

Health officials warn that food tainted with salmonella may not look or smell bad, but it can make you sick nonetheless. Symptoms include diarrhea, fever, vomiting and abdominal cramps that usually appear 12 to 72 hours after eating contaminated food. They can last up to seven days.