What is cancer? While cancer is usually thought of as one disease, in reality the term is used to identify over 100 diseases that have similar characteristics. Your body is full of cells that reproduce themselves by dividing in a regulated and controlled manner. Cancer occurs when normally dividing cells, due to the effect of carcinogens (cancer-causing chemicals), become uncontrollable and do not stop reproducing. The result is too many abnormal cells that are not functioning properly. These cells continue reproducing, and they act as a parasite in the body, stealing nutrients and strength from the healthy cells around them. Because the cells reproduce haphazardly without order, they usually form a noticeable growth, called a tumor, which then can spread very rapidly.

Cancer cells are able to spread to distant parts of the body by traveling through the blood stream or the lymphatic system. Each kind of cancer behaves differently based on the organ it started in, even after it spreads to other parts of the body. For example, breast cancer cells that have traveled to the lungs still behave like breast cancer, not like lung cancer.

Because of cancer’s ability to spread quickly, the best chance of success in treating it is to catch it very early on, before it has grown too large or spread to other parts of the body. If the cancer is still in a concentrated area, it can sometimes be completely removed by surgery. More often, however, chemotherapy is used in addition to surgery.

Chemotherapy is usually associated with cancer treatment, although any chemical treatment is technically chemotherapy, even common cold medicine. While cold medicine or antibiotics attack the harmful pathogens in your body, such as viruses or bacteria, cancer chemotherapy uses chemicals to kill cancerous cells. Because only reproducing cells are susceptible to the chemicals, chemotherapy mostly affects cancer cells, which are continually reproducing. In an adult, most of the cells in the body are not reproducing on a regular basis, unless they are repairing damage from injuries, or replacing worn out cells. Because of this, chemotherapy attacks the cancerous cells and does not harm the healthy cells in the body.

There is one major exception, however. The healthy cells that make up certain parts of the body are continually reproducing, and therefore are also affected by the chemotherapy. This is where the side effects of chemotherapy come from — hair is always growing; so the chemotherapy attacks the reproducing hair cells, and the patient loses their hair. The lining of the digestive system is also constantly replenishing itself; the chemotherapy attacks those reproducing cells, which is why patients often experience intense nausea. And, most dangerously, the white blood cells which are so important to the immune system are also attacked by the chemotherapy, making the patient susceptible to other sicknesses.

In spite of these difficult side effects, chemotherapy has saved many lives from cancer. The healthy cells of the body are able to recover much easier than cancer cells, so the side effects usually disappear after the treatment is over. There is still much to be discovered about cancer, but as scientists and doctors gain more knowledge, they are able to treat it more and more effectively.

To see the difference between healthy cells and cancerous cells, you may want to compare these two microscope slides: one is a the section of a healthy lung, and the other is from a cancer-infected lung.