Risk factor
Anything that raises the chance of developing a cancer: smoking, alcohol, obesity, sunlight, certain infections, inherited genes, age. Having a risk factor does not mean getting cancer, and many cancers occur without any known one.
Around 40% of cancers in high-income countries are attributable to modifiable factors, tobacco alone causing about a fifth of cancer deaths, followed by excess weight, alcohol, infections (HPV, hepatitis B and C, H. pylori), ultraviolet light, and inactivity; the IARC classifies agents by the strength of evidence that they cause cancer in humans. Non-modifiable factors include age (the biggest), sex, inherited mutations, and family history. Risk factors drive prevention (vaccination, smoking cessation, alcohol policy), define who is offered screening (heavy smokers for lung CT, BRCA carriers for MRI), and, through the mutational signatures they leave, can sometimes be read directly from a tumour's DNA.