A virus known to cause leukemia and tumors in animals can be found in some prostate tumors and might be one cause of prostate cancer, U.S. researchers reported on Monday.

They found xenotropic murine leukemia virus-related virus or XMRV in 27 percent of the human prostate tumors they looked at, especially aggressive tumors.

The findings, published in the Proceedings of the National Academy of Sciences, may offer ways to better identify dangerous prostate tumors and to make drugs or vaccines to treat or even prevent prostate cancer.

Our analysis of 233 cases of prostate cancers and 101 benign controls showed an association of XMRV infection with prostate cancer, especially with more aggressive tumors, Dr. Ila Singh of the University of Utah and Columbia University in New York and colleagues wrote.

Viruses have recently been found to cause some cancers -- notably the human papillomavirus or HPV that causes cervical cancer and some cases of penile, anal and head and neck cancers.

Merck and Co. and GlaxoSmithKline now make vaccines to prevent HPV infections.

The market for a vaccine to prevent prostate cancer or for better drugs to treat it could be enormous. Prostate cancer is the second most common cancer in men worldwide after lung cancer, killing 254,000 men a year globally.

The lifetime risk for developing prostate cancer is one in 6 in the United States, and globally, 3 percent of men die of prostate cancer, Singh's team wrote.

The researchers were scanning for genetic material from any viruses in prostate tumors.

We found that XMRV was present in 27 percent of prostate cancers we examined and that it was associated with more aggressive tumors, Singh said in a statement.

We still don't know that this virus causes cancer in people, but that is an important question we're going to investigate.

XMRV is a retrovirus, a kind of virus that inserts its genetic map into the cells it infects.

This act alone can kill a cell or turn it cancerous by affecting its genes. Singh said she was looking to see if this might be going on with XMRV.

Is the virus associated with cancers in tissues other than the prostate or in gynecologic malignancies? How is XMRV transmitted? These are all intriguing questions that deserve further exploration, the researchers wrote in their report.

The report is available online here

Just last month, U.S. researches estimated that 1 million U.S. men had been diagnosed and treated for tumors that would never have harmed them.

Many cases of prostate cancer are unlikely to manifest themselves during the patient's lifetime. There is a clear need for better markers to detect cancers that pose a significant health threat and to specifically target these for therapy, Singh's team wrote.