Can you get cancer even with no known risk factors?

Yes. Cancer can develop in some people who do not have any known risk factors. It is usually not possible to know exactly why one person develops cancer and another does not.

The reason sits in how cancer starts. Cancer is a genetic disease, caused by changes to the genes that control how cells grow and divide. Those changes arrive by three routes. Some are inherited from a parent. Some come from damage caused by things in the environment, such as chemicals in tobacco smoke or ultraviolet rays from the sun. And some are simply errors that occur as cells divide.

That third route is the answer to this question. Cell division is copying, and copying produces mistakes. Your body does this constantly, in enormous numbers, for your whole life. No exposure and no habit is required. A share of cancers trace back to copying errors that nobody caused and nobody could have prevented.

Age makes those errors matter more. The body normally clears out cells with damaged DNA before they turn cancerous. That ability declines as we get older, which is part of why cancer risk rises later in life. It is also why almost nobody truly has zero risk factors. Age counts, and everyone has one.

A risk factor is anything that may increase a person's chance of developing cancer. But risk factors change the odds; they do not decide what will happen to any one person. Having one or more risk factors does not mean a person will get cancer, and cancer can happen even when no known risk factor is present.

It also helps to know how risk factors get identified in the first place. Most come from epidemiology studies, where scientists follow large groups and compare those who develop cancer with those who do not. On their own, those studies cannot prove that a behavior or substance causes cancer. A finding could be chance, or the real cause could be something else that traveled alongside the suspect. Confidence grows only when many studies point the same way and there is a believable biological explanation for how the factor could cause cancer. That is why headlines about a food or product seem to reverse every few years, while the well-established causes never move.

NCI's list of the most-studied known or suspected risk factors is short enough to read in a breath: age, alcohol, cancer-causing substances, chronic inflammation, diet, hormones, immunosuppression, infectious agents, obesity, radiation, sunlight, and tobacco.

Some risk factors, such as certain exposures and behaviors, can be avoided, while others, such as age and family history, cannot. Limiting your exposure to avoidable risk factors may lower your risk of certain cancers. Your healthcare team can help you understand your own situation.

Two things follow from all this. If you have cancer and cannot find a cause, the honest answer is often that there is not one to find, and hunting for a personal failure is wasted energy you will need elsewhere. And if you have no risk factors, that is not a reason to skip screening or to sit on a symptom. Recommended screening applies to people at average risk, which is most people, and a new lump or unexplained bleeding deserves a look regardless of how clean your history is.