CERVICAL cancer is usually caused by a viral infection. Two particular strains of the human papilloma virus (HPV) are responsible for 70% of such cancers. They do so by turning off genes that would otherwise suppress the formation of tumours. HPV infections do not, however, necessarily lead to cervical cancer—the best estimate is that less than 1% do. And a paper just published in the Lancet Oncology by Xavier Castellsagué of the Catalan Institute of Oncology, in L'Hospitalet de Llobregat, Spain, and his colleagues suggests a way even that figure might be reduced.

Dr Castellsagué was intrigued by evidence that intrauterine devices (IUDs)—small, T-shaped gadgets that release hormones or copper to prevent pregnancy—reduce the risk of endometrial cancer. The devices' effects on cervical cancer have been less clear. Indeed, since the 1930s, some researchers have speculated that IUDs might actually increase the chance that cancer will develop. Dr Castellsagué thought the matter worth investigating.

He and his colleagues therefore analysed data from two large studies on HPV and cervical cancer. These studies spanned four continents, two decades and more than 20,000 women. IUDs seemed to have no impact on the rate of HPV infection per se. However, a woman's risk of developing cervical cancer dropped within a year of IUD insertion to half that of one who had never used an IUD. What is more, this protective effect remained the same after a decade of use. The authors controlled for a range of characteristics, such as marital status, number of sexual partners and number of Papanicolaou smear tests. Their findings were unchanged by such controls.

The evidence for IUDs' effect on cervical cancer is compelling. How the devices thwart HPV's progress, though, is a mystery. Dr Castellsagué offers a few possible explanations. An IUD's hormones might play a role. The process of inserting an IUD might scrape away precancerous lesions. Or (and he thinks this is the most likely explanation), an IUD may cause inflammation and thus prompt an immune response that helps to fight HPV infections. Biopsies (removals of live tissue for medical examination) are known to have a such a suppressive effect on other infections. Dr Castellsagué's paper adds to the evidence that medical tinkering of this sort can have unexpected benefits.