Cancer is the out-of-control growth of cells in your body. Cells are the tiny building blocks of your body. Cells specialize in what they do. For example, your intestines have muscle cells to make them contract, nerve cells to control the muscle cells, and other cells to absorb food. There are many more types of cells in the intestines and in other parts of your body.
Normally, new cells in your body grow only to replace cells that have died or gotten too old. Each new cell looks and functions just like its parent cell. Cancerous (malignant) cells are different from normal cells in many ways because they:
Some kinds of cancer cells group together to form a solid mass called a tumor. However, not all tumors are cancerous. Tumors that aren't cancerous are called benign (harmless).
When doctors talk about the “stage” of cancer (stage I, stage II, stage III, or stage IV), they're describing how big the cancer is and if and where it has spread. Some cancers grow and spread faster than others. These are called aggressive cancers.
If you're healthy and gain 15 or 20 pounds of weight, you'll probably feel fine. However, even a few pounds of cancer cells can be enough to kill you if they:
Dangerous places for cancer to grow include:
Lungs: Trouble breathing
Belly: Blocking your intestines
Brain: High pressure inside your skull
Major blood vessels: Heavy or fatal bleeding
Bone marrow (the hollow insides of your bones, where blood cells are made): Dangerously low blood count (anemia), excessive bleeding, trouble fighting off infections, weak bones that break easily
The substances cancers give off can cause problems even when the cancer isn't growing in a dangerous place. Some of the substances do the following:
Take away your appetite: severe weight loss, trouble fighting off infections
Interfere with making blood cells: low blood count (anemia), excessive bleeding, trouble fighting off infections
Interfere with your body's chemical balance: abnormal heart function, coma
The problems caused by the substances are called paraneoplastic syndromes.
Cancer isn't one disease, but many.
The 5 most common cancers in men, from most to least common:
The 5 most common cancers in women, from most to least common:
That said, skin cancer is probably the most common. However, because accurate counts aren't available for all types of skin cancer, skin cancer isn't included on these lists.