There is growing evidence that certain types of bacteria may cause some cancers, according to a leading expert.
It is a relatively new area of cancer research
Alistair Lax, professor of cellular microbiology at King's College London, said bacteria may be involved in stomach, renal and bowel cancers.
He said testing for certain bacteria could enable doctors to identify those at risk of cancer much earlier.
Professor Lax was speaking ahead of a Society for General Microbiology meeting in Bath next week.
He is scheduled to tell scientists that there is now evidence to suspect bacteria in a range of cancers.
He will highlight recent studies into Helicobacter pylori. As many as one in three people may carry this bacteria, which can cause stomach ulcers.
But many scientists now believe it may also play a role in stomach cancer.
Professor Lax said this and other bacteria may interfere with cells in the body, causing them to turn cancerous.
Cells in the body are constantly changing. They are growing, dividing or dying off.
However, poisons produced by bacteria can interfere with the communication process that regulates this cycle.
Some scientists believe that they can cause cells to mutate and turn into a cancer cell.
"Some of these bacterial poisons have properties that may be involved in some cancers," he told BBC News Online.
"It is not every bacteria and it is not every cancer but in some cancers there is growing evidence to suggest some bacterial toxins could have a role."
A growing number of scientists are carrying out research in this area.
Cancer Research UK is funding a large-scale trial to see if screening for H.pylori can identify those at risk of developing cancer.
Professor Nicholas Wald at the Wolfson Institute of Preventive Medicine is hopeful that the trial will be successful.
"H.pylori infection is now accepted as a cause of stomach cancer," he told BBC News Online.
"What we are trying to do is to see if treating this infection can reduce the risk of stomach cancer.
"The evidence suggests it is likely to be. Our study will end in a couple of years."