What causes cancer?

Things people do

Some kinds of cancer are caused by things people do. For example, smoking can cause cancers of the lungs, mouth, throat, bladder, kidneys, and other organs, as well as heart disease and stroke. While not everyone who smokes gets cancer, smoking increases a person's chance of getting the disease.

Being in the sun for extended periods of time without protection can cause skin cancer.

Things people are exposed to

Radiation can cause cancer. For example, people exposed to nuclear fallout have a higher cancer risk than those who were not exposed. Rarely, radiation treatment for one type of cancer can cause another cancer to grow many years later. This is why doctors and dentists use the lowest possible doses of radiation for x-rays and scans (much lower than that used for cancer treatment). Certain chemicals have been linked to cancer, too. Being exposed to or working with them can increase a person's risk of having cancer. Of every 20 cases of cancer, about 1 is caused by genes that are inherited from parents.

No one knows the exact cause of most cases of cancer. We know that certain changes in our cells can cause cancer to start, but we don't yet know exactly how it all happens. Scientists are studying this problem and learning more about the many steps it takes for cancers to form and grow. Although some of the factors in these steps may be a lot alike, the process that happens in the cells is generally different for each type of cancer.

Can injuries cause cancer?

No. It is a common myth that injuries can cause cancer. The fact is that a fall, a bruise, a broken bone, or other such injury has not been linked to cancer. Sometimes a person might visit the doctor for what is thought to be an injury and cancer is found at that time. But in cases like this, the injury did not cause the cancer; it was already there. It also sometimes happens that a person will remember an injury that happened long ago in the place cancer was found.

Rarely, burn scars can be the site of cancer many years after the burn has healed. Most often, skin cancer is the type that grows in a burn scar.

Can stress cause cancer?

Researchers have done many studies to see if there is a link between personality, stress and cancer. No scientific evidence has shown that a person's personality or outlook can affect their cancer risk.

There are many factors that come into play when looking at the relationship between stress and cancer. While it is known that stress affects the immune system, so do many other factors. Despite many studies, a link between psychological stress and cancer has not been proven. Looking at the studies that have been done, it seems they sometimes come to opposite conclusions.

In one large Danish study, people who reported major stress in their lives did not appear to have a seriously increased risk for any type of cancer. Another study that looked at women with major life stress, such as divorce or the death of someone close, found a slight increase (about 1/3 higher than average) in breast cancer compared to women without these stress. In the area of day-to-day stress, another study showed higher breast cancer risk linked to stress. Yet another found that women reporting higher day-to-day stress actually were less likely to be diagnosed with breast cancer within the next 18 years. It is hard to explain these differences. Some may be related to the groups that were studied, while others may be due to the way the study was done. Chance may have played a role, too. All that can be said for now is that a definite link between stress and cancer risk has not yet been found.

Is cancer contagious?

In the past, people often stayed away from someone who had cancer. They were afraid they might "catch" the disease. But cancer is not like the flu or a cold. You cannot catch cancer from someone who has it. You will not get cancer by being around or touching someone with cancer. Don't be afraid to visit someone with cancer. They need the support of their family and friends.