In a normal inflammatory response, immune cells produce chemicals that can kill a pathogen. These chemicals, known as reactive oxygen species, can also damage the DNA of normal cells, which increases the risk of mutations that could lead to cancer.

"Timing is everything," said Jennifer Kay, Ph.D., of the Silent Spring Institute, who studies how healthy cells become cancerous. "If the optimal timing of biological processes related to inflammation is altered, the chances of cancer occurring increase."

For instance, in the normal inflammatory response, the production of cells to replace injured tissue is normally delayed until reactive chemicals are no longer being produced. This sequence of events reduces the chances that replacement cells will sustain DNA damage, including cancer-causing genetic mutations, caused by reactive chemicals.

But during chronic inflammation, the production of reactive chemicals can overlap with the production of cells that restore injured tissue, Dr. Kay noted. This can potentially increase the risk of cancer.

The reasons inflammation starts when it is not needed or becomes chronic are not always clear. Some recent studies have focused on the failure of mechanisms that normally shut down inflammation at the appropriate times.