Canadian researchers have uncovered a brand-new dimension to an infectious disease older than the Bible.

An international team led by scientists from Montreal's McGill University are homing in on a gene that makes certain people susceptible to leprosy, the disfiguring disease triggered by a bacterial infection.

Officials with the World Health Organization have for decades pushed the view that the mycobacterium leprae, which strikes more than half a million people a year worldwide, can infect anyone. In part, they hoped to discourage the intense discrimination leprosy victims suffer, from the historically ostracized "leper colonies" to modern-day death threats.

But this finding, published today in Nature Genetics, suggests that some people are, in fact, less equipped than others to battle this ancient bug.

"There has been a very dedicated effort to dispel the notion that leprosy has this genetic component because it has this enormous social stigma," said McGill geneticist and co-author Erwin Schurr.

"But this shows you, you cannot reject communal wisdom, even if it does not lead to appropriate behaviour."

The work also raises the possibility that scientists will be able to find susceptibility genes that make people vulnerable to a range of common infections, such as tuberculosis. But some experts feel such genes will be very tough to find.

Still, Dr. Schurr said, "As we understand genetic vulnerabilities, we will be able to develop more effective treatments, specifically new vaccines."

Scientists have already found other genes that can protect people against or predispose them to certain diseases. For example, 1 per cent of Caucasians carry a trait that makes them immune to HIV. Many people are believed to possess a form of a gene that delays the onset of the human form of so-called mad cow disease. And others are genetically vulnerable to malaria.

Geneticist Steve Scherer, a senior scientist at Toronto's Hospital for Sick Children, said some researchers suspect that genes will turn out to explain a range of common diseases that are not generally considered to be infectious.

"It could be that Crohn's disease, irritable bowel disease, or even psychiatric illnesses are the result of infections that interact with [susceptibility] genes," Dr. Scherer said.

But these genes can be tricky to find, he explained, because scientists are not necessarily looking for a gene mutation, or a defect, but a mysterious trait that makes people susceptible.

Dr. Schurr, working with Tom Hudson of the Montreal Genome Centre and scientists in France and Vietnam, identified the genetic traits linked to leprosy in 86 families from South Vietnam that had 205 members affected by the disease.

The four-year study, backed by the Canadian Institutes of Health Research, compared the DNA of these families with another 208 Vietnamese families that had only one member affected by leprosy.

The researchers are now investigating whether these same traits appear in the general population, particularly in India and Brazil, which suffer high leprosy rates. The disease is rare in Canada, as it is in most industrialized countries.

"The data sound convincing that there is a susceptibility gene there," Dr. Scherer said. "My guess is that they will find this in the general population."

Leprosy, which attacks the skin and peripheral nerves, particularly in the fingertips and toes, and mucous in the upper respiratory tract and eyes, can be effectively treated with chemotherapy.

But because the bacteria trigger an autoimmune response, irreversible nerve damage can continue even after the infection is cured.

Dr. Schurr suspects there are also genes that protect people from developing leprosy, but allow them to carry the m. leprae bacteria and pass it on to others.