A common virus found in sewage may trigger colon cancer, a study suggests.
Researchers in the United States have found the JC virus (JCV) in cancer tumours.
The virus is carried by 90% of people but is usually harmless.
The researchers say further study is needed to see if there is a direct link between JCV and colon cancer.
But they believe that the discovery could help in the development of new treatments to protect people from the disease.
JCV is generally contracted during childhood but lies dormant.
However, it can cause problems in patients with depressed immune systems.
The virus is already known to cause a deadly disease of the nerves called progressive multifocal leukoencephalopathy (PML) – particularly common in Aids patients.
Recent studies have suggested that it can be transmitted through contaminated water and food putting people at risk of re-infection. It is also found in sewage.
While the virus usually remains in the respiratory tract, re-infection can cause it to spread to other organs.
Professor Kamel Khalil and colleagues at Temple University in Pennsylvania have found evidence to suggest that it can be found in the intestine.
They have also found evidence that it releases proteins that can trigger cancer tumours.
The researchers analysed both cancerous and non-cancerous tumours from the colon of 27 patients.
They found traces of JVC in 22 of the 27 cancerous tumours.
Professor Khalili said: “We have a virus in our body which may be involved in causing tumours.”
But he added that further study is needed to see if there is a direct link.
“We are not saying every single tumour is called by the JC virus, because when you detect a virus in a tumour you cannot actually say the virus causes it.
“What we are stating is that the virus can be detected in a good number of the human colon cancer tumours we sampled.”
Professor Khalili said the finding could strengthen the case for scientists to start trying to develop a vaccine or treatment to combat JCV.
“We can start developing strategies and vaccines against JC virus, which will hopefully lead to the prevention of the tumours it may induce.”
Previous studies by the Temple University researchers have linked the virus to brain tumours.
The study is published in the journal Cancer Research.