The researchers all agree smoking is by far the most important risk factor for lung cancer, but Dr Arash Resazadeh and colleagues from the University of Louisville, Ky., found five out of 22 non-small-cell lung cancer samples tested positive for human papilloma virus.

"We think HPV has a role as a co-carcinogen which increases the risk of cancer in a smoking population." Resasadeh says in a statement.

In another study, Samuel Ariad from Soroka Medical Center in Beer Sheva, Israel, and colleagues found more than half of the 65 patients with non-small-cell lung cancer had evidence of measles virus in tissue samples taken from their cancer.

In his study, Ariad found the "measles virus is a ubiquitous human virus that may be involved in the pathogenesis of lung cancer. Most likely, it acts in modifying the effect of other carcinogens and not as a causative factor by itself."

Both studies have been presented at the 1st European Lung Cancer Conference in Geneva, Switzerland, jointly organized by the European Society for Medical Oncology and the International Association for the Study of Lung Cancer.