In the 1990s, Dr. John Dick, a professor at the University of Toronto, found cancer stem cells in leukemia.

It used to be that scientists and doctors thought of cancer as just an amorphous blob of uncontrollably dividing cells. Now they are starting to believe that all cancer cells are not created equal and we should focus our attention on a particular type: the cancer stem cells.

Peter Dirks is a brain surgeon at Toronto's Hospital for Sick Children who has been studying cancer stem cells for several years. He's found that they are not only much more difficult to fight than regular cancer cells, they are also much more nefarious.

Jessica Durigon, a young girl who died just two days after her first birthday, is a perfect example of why.

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Born on March 19, 2002 to Wendy and Rob Durigon, Jessica's early life was, as most parents hope, normal. She was slowly growing up and, in the process, becoming her own person. She was never a happy baby the way you think of happy babies. She did smile, a little, but even before her diagnosis, she was serious. Her mother said she used to think she had "old eyes."

When Jessica was only four-and-a-half months old, she was diagnosed with brain cancer.

Dr. Dirks was the surgeon who operated on little Jessica. The surgery appeared to be a complete success and several weeks later, there was no sign whatsoever of the cancer. Jessica was slowly getting better and the Durigons were beginning to relax again. Unfortunately, Jessica's cancer returned. This time, it was inoperable.

Through the pain of her death, the Durigons have chosen to celebrate Jessica's life by fighting for other children. The Durigons and other families are helping Dr. Dirks uncover why certain cancers, which look completely cured, come back. Dr. Dirks thinks the answer lies in the cancer stem cells.

Cancer stem cells aren't new. In the 1990s, Dr. John Dick, a professor at the University of Toronto, found them in leukemia. From his experiments, he was able to show that about one leukemia cell in a million was a cancer stem cell. It was these rare cancer cells, not their much more common offspring, that seemed to drive the leukemia. What happens in other cancers is still a matter of debate but cancer stem cells have been found or are suspected to exist in breast, brain, lung, prostate and several other forms of cancer.

Stem cells have been found in most parts of our bodies. Stem cells act as an important repair system because of their ability to replace and replenish damaged cells. But what makes a stem cell special is that as it divides also make a perfect copy of itself to keep the process going over and over again.

Cancer stem cells work in much the same way, replacing and replenishing other cancer cells but also making perfect copies of themselves to keep the process going.

Cancer therapy is designed to kill cancer cells that divide uncontrollably but killing cancer stem cells seems to be a bit trickier. Scientists suspect that cancer stem cells just don't divide fast enough to be caught by our current cancer therapies. This means that cancer stem cells can stay in the background and cause the cancer to re-emerge later, even if the bulk of a cancer was killed off. It's still unclear whether some cancers actually arise in stem cells but that doesn't change the fact that cancer stem cells appear to be what keeps the cancer going.

Dr. Dirks has seen how conventional cancer therapy can fail his patients. He will use the research funding from families like the Durigons to identify and isolate these brain-cancer stem cells and hopefully find a better way to kill them. But it's still the early days of research, he cautions. Even though the idea of cancer stem cells is starting to be accepted, there is still a lot that scientists and doctors don't know about them.

While the ultimate goal is to create therapies that can specifically target these cancer stem cells, the reality of doing this may be very complicated. "I think that the cancer research enterprise is going to need to be rejigged to take account for these cancer stem cells," says Dr. Dick. That rejigging is the focus of his, and Dr. Dick ongoing research.