An American chimpanzee named Marilyn has helped answer a very important question.

Scientists studying samples of Marilyn's blood say they can now prove that the virus that causes AIDS first passed from chimps to humans.

Of course this is interesting to researchers. But what's exciting is that this new knowledge could be a giant step towards finding a cure for a disease that affects thousands of Canadians and millions of people around the world. What's interesting is not that Marilyn had AIDS -- but rather that she didn't. Even though Marilyn was infected with a virus that is extremely close to the HIV virus that causes AIDS in humans, she didn't get the disease. Marilyn didn't even get sick.

Scientists want to know why not. That knowledge may help them find ways of preventing AIDS in humans. Dr. Beatrice Hahn of the University of Alabama in Tuscaloosa, Alabama and colleagues made the discovery when analyzing Marilyn's blood and tissue samples after she died at the age of 26. She had spent her life in a lab.

"She had never been used in AIDS research and had not received human blood products after 1969," says Dr Hahn. "She died in 1985 after giving birth to still-born twins."

There have been many competing theories about where HIV comes from, but most scientists believed the virus must have first come from apes or monkeys.

Humans are the only creatures that can get HIV, which stands for human immunodeficiency virus. But apes and monkeys can get a ``simian immunodeficiency virus'' or SIV. But until now, only three cases of chimpanzees infected with SIV had been documented.

When Dr. Hahn's team found the virus in Marilyn, they compared it to the other SIV viruses and to several strains of HIV. The SIV strains taken from the rare west African chimps, known as ``Pan troglodytes troglodytes'' and including Marilyn, very strongly resembled the three subgroups of HIV .

"This is an important finding with significant potential," says Dr. Anthony Fauci of the U.S. National Institute of Allergies and Infectious Diseases.

`` This virus infects a primate species that is 98 percent related to humans. This may allow us to study infected chimpanzees in the wild to find out why these animals don't get sick, information that may help us better protect humans from developing AIDS," he says.

Many viruses come from animals. Flu, for example, comes from ducks and pigs. Often the virus does not make its natural host, or reservoir, sick. It must undergo genetic changes to infect another species such as humans, changes that can cause illness.

Dr. Hahn's team found evidence of such genetic changes in their chimp SIV, known specifically as SIVcpz .

Last year, researchers said they had found the first known case of AIDS - in a Bantu man who died in 1959 in a country then known as the Belgian Congo. It is now the Democratic Republic of Congo.

That is where this rare subspecies of chimp lives.

Many scientists think the virus spread to people through infected ape or monkey meat . People in some parts of Africa often hunt and kill chimpanzees for food . A virus can easily spread from blood during butchering.

1. Who wants to know why not?