Written by Science Knowledge on 12:17 AM

So far, you’ve spent most of your time exploring battles that are relatively clear-cut. They pit your body against outside forces, like invading bacteria and viruses. But now it’s time to consider a subtler enemy—rogue cells in your own body.

The disease is cancer, and it starts with a subtle genetic shift that transforms one of the trillions of ordinary cells in your body into a saboteur. Unlike the cells in the rest of your body, a cancerous cell isn’t bound by the normal rules of human life. It grows without respect for the boundaries between different types of tissue, invades other sites in the body, and refuses to die a natural death. In the end, what begins as a series of simple cellular errors can become an unstoppable process that ravages your body.

How Cancer Starts
Although we often imagine cancer as a single thing, it’s actually a family of diseases that’s characterized by misbehaving cells. The problem begins with a chance mutation in a key regulatory gene—essentially, a cell turns off one of the safeguards that restricts it or over-activates one of the mechanism that drives normal cell growth. However, a single mutation isn’t enough—if it were, you’d be riddled with cancer while you were still in diapers. Instead, cancer needs to develop through a succession of highly improbable mutations, which gradually give the cell and its offspring the ability to defeat several different control mechanisms.

The picture on the next page shows one way this process can unfold:
1. The cell begins its life as normal.

2. Random mutations give the cell the ability to ignore the normal recycling processes of your body, so instead of dying, the cell lives forever. This transformation happens quietly and without event.

3. Next, the cell multiplies, creating similar ill-tempered progeny and crowding healthy cells out of the way. This unchecked growth often creates an abnormal mass of tissue called a tumor, which can cause problems if it presses on one of your vital organs.

4. The real trouble with cancer occurs when the cancerous cells metastasize, or spread to other areas of your body. Once cancer cells have become mobile, they travel far and wide, voyaging through your blood and lymph, and starting new cancer settlements throughout your body. At this point, the odds of successful treatment dwindle quickly.

Because different cancers acquire different mutations, they vary in their virulence. The nastiest forms multiply quickly and travel aggressively, and they can rapidly colonize your body. Other forms are highly treatable and have better survival rates than a heart attack or stroke.

When diagnosing a new cancer in a patient, doctors classify how far it’s advanced by stage. The exact definition of the various stages (and the prognosis of a cancer patient) depends on the type of cancer and its location. But in general, stage I cancers have not yet spread and are usually treatable. Stage II cancers have had some time to develop but have not yet traveled the body, while stage III cancers have made it to nearby lymph nodes. Stage IV cancers are the worst— they’ve spread to organs throughout the body and are usually untreatable.

Ordinarily, cells have a built-in self-destruct sequence. When a cell detects that it’s diseased or damaged (or when other cells detect something suspicious and convince the cell that it’s not quite right), the cell initiates this self-destruct sequence and destroys itself in a calm and orderly fashion. This tidy suicide process is called apoptosis, and it’s as fundamental to the functioning of your body as cell division. However, successful cancer cells don’t obey the shutdown command— they stay alive, multiply, and can develop more dangerous mutations.

Source of Information : Oreilly - Your Body Missing Manual

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In its broadest sense, science (from the Latin scientia, meaning "knowledge") refers to any systematic knowledge or practice. In its more usual restricted sense, science refers to a system of acquiring knowledge based on scientific method, as well as to the organized body of knowledge gained through such research.

Fields of science are commonly classified along two major lines: natural sciences, which study natural phenomena (including biological life), and social sciences, which study human behavior and societies. These groupings are empirical sciences, which means the knowledge must be based on observable phenomena and capable of being experimented for its validity by other researchers working under the same conditions.

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