They report today in the Journal of the National Cancer Institute that they chose an adenovirus - a virus of the respiratory tract - which targets only cancerous cells, and which spreads like a wave through malignant glioma, the most lethal form of brain cancer.

They injected it into laboratory mice implanted with human glioblastoma tumours. Untreated mice died in three weeks, but treated mice survived for more than four months. When they examined the survivors, investigators found only empty cavities and scar tissues where the brain tumours had been.

"We believe this therapy has a lot of potential, but one that needs much more study," said Juan Feyo of the University of Texas MD Anderson cancer centre. "We've never seen this kind of response before with any other treatment tested in either animals or humans."

The US National Cancer Institute is to produce a drug to test in humans late in 2004. The virus, known so far by the clumsy name of Delta-24-RGD, replicates only in cancer cells, not healthy tissue.

It reproduces itself by killing its host cells and moving on to contaminate other tumour cells. When there are no more cancer cells to infect, it dies.

As there is no successful treatment for malignant glioma, the researchers have in effect turned an unstop pable infection into a weapon against an incurable disease.

"Cancer can be devious, in that it does everything possible to evade destruction. But viruses are equally tricky in their quest to invade cells and propagate," said Frederick Lang, one of the investigators. "In this experimental war between cancer and a viral therapy, the virus won. Of course, we hope to obtain similar results when patients are tested, but we cannot predict such success based on animal studies."

Viruses hijack their hosts' DNA to multiply. Cancer is above all a DNA disease, and the cancer genome project in Cambridge has found more than 100 genetic mutations associated with cancer. The reasoning is that if humans can identify a cancer cell by its DNA, then so can a virus.

Researchers have proposed an altered HIV virus for gene therapy in haemophilia, cystic fibrosis and heart disease. Other teams have experimented with bacteriophages - viruses that prey on bacteria - to defeat antibiotic-resistant infection. Cell biologists experimented with an altered virus to deliver growth factor to wasting muscles, and mutant common cold viruses have been used to shrink tumours in prostate cancer and head and neck cancer.

But most experiments have been with mice, and most are rated only "promising" so far. The cooperation between the University of Texas, the University of Alabama at Birmingham, and the Catalan cancer institute in Barcelona, Spain, has achieved more than that.