Cancer is a disease characterized by the aggressive, spreading growth of abnormal tissue, concentrated into lumps known as tumors.

Cancer occurs from a genetic mutation in a cell or a group of cells that causes the process known as apoptiosis to become disrupted, resulting in cells that 1) never cease reproduction, and 2) are never destroyed/recycled properly. This causes normal, healthy organs and blood vessels to become both slowly compressed and converted to cancer tissue. This can lead to organ failure, and abnormal cell death (known as necrosis). The cells themselves can migrate to the bloodstream or lymph glands, and spawn tumor growth in other areas, a process known as Metastasis.

Symptoms include fatigue, weight loss, (as the tumor prioritizes energy and nutrient intake away from healthy tissue), pain (a general signal of "something has gone wrong"), bleeding (blood vessels being crushed or degraded by abnormal tissue), difficulty going to the bathroom (the colon or bladder being damaged), and lumps (the malignant growth itself). Later stages are characterized by rapid metastasis into other organs, and gangrene as starved carcenomas die and become necrotic.

The treatment is as bad as the disease. A regimen of drugs, known as Chemotherapy, supress the growth of the tumor by ceasing cell division, causing stomach pain, and loss of hair and weight. Basically, poisoning the patient in the hopes that the abnormal, mutant tissue dies before healthy tissue does. There is also radiotherapy, which involves exposing the patient to doses of gamma radiation concentrated on the growth. Since radiation kills the tumor by mutating it to the point that it can't reproduce any more, it is used sparingly so it won't make the problem even worse.

A similar affliction is called Teratoma, in which normal organ tissue begins growing in other parts of the body, for example, a kidney suddenly growing in the liver.