Cancer is a class of diseases or disorders. It is when the body has no control over cells that begin to split. In cancer, body cells copy their contents. They then make new cells with these copies. These cells are able to go into other tissues. They go into other tissues by growing into them. They can also go into other tissues by putting themselves into far away places by metastasis. Metastasis is the stage in which cancer cells move through the bloodstream or lymphatic system. Cancer can affect anybody at any age. People are more likely to get it as they get older. This is because DNA damage becomes more apparent in older DNA. An exception is testicular cancer. It is more common in young men. Cancer is one of the biggest and most researched causes of death in developed countries.

Cancer is a leading cause of death. It causes about 12.5% of all deaths worldwide. This is according to the World Health Organization. There are some things that can be causes or triggers of different types of cancer. They include tobacco (smoked or smokeless), marijuana, lots of sunlight, radiation (including X-rays in large or many doses and exposure to radiation in a nuclear power plant), chemicals used in building and manufacturing (for example, asbestos and benzene), high-fat or low-fiber diets, air and water pollution, people who eat very little fruits and vegetables, obesity, not enough physical activity, drinking too much alcohol, and household use of some chemicals. Some cancers can also be caused by viruses.

There are lots of different kinds of cancers. Some of the most common include breast cancer, brain cancer, leukemia, testicular cancer, mesothelioma, and lung cancer. Breast cancer begins in the breast. It can be found in anybody at anytime. It can also be found in men. Brain cancer starts in the brain (cancer of the brain is called a brain tumour). Testicular cancer starts in the testicles. It is most common in young men. Mesothelioma and lung cancer start in the lungs.

There is no cure for cancer. Cancer can sometimes be treated using radiotherapy (also called radiation therapy), chemotherapy, or immunotherapy (also called biological therapy).

One hard problem in treating cancer is that most things that kill cancer also kill normal cells. Cancerous cells are often quite weak. Since the cancer is actually a part of the body, the body will not attack it, even though the cancer often could easily kill the body. Another hard problem in treating cancer is that there are many different types of cancer. Each have their own symptoms and causes. Some people can handle cancer better than others. However, it is harmful to everybody. It can be fatal.

There is always progress in finding treatment. Many cancers are treated successfully. For example, a study showed that if someone with lymphedema (a swelling of the arm linked to breast cancer) lifts weights, they be able to fight cancer better than somebody who doesn't.