We are made up of around 30 trillion cells, although we start from a single cell. This cell replicates itself, and each of the copies replicate themselves, and this happens over and over again. Eventually these cells form a complete human being.
Early cells, also called Stem cells, are undifferentiated. This means that they have the potential to become any kind of cell, but haven't specialised. Once a certain number of cells have been formed, new cells begin to differentiate, and become unique cells, with very a specific purpose.
Cells come in around 200 different types, and each has a special role in the body. They have different sizes and shapes, and even different life spans.
When cells die or are damaged they need to be replaced, so cell division is a process that continues throughout our lives. Each day, the human body makes about 300 billion new cells. More than half of these are red blood cells, which only last for around 120 days, and are constantly being replaced. This is a completely normal process.
With a couple of important exceptions, when healthy cells divide, they are pre-programmed to become a specific sort of cell, and there are controls about whether they can replicate themselves. They also have a programmed age limit, after which they naturally die and are replaced.
A tumour occurs when there is a mistake in the copying process, and the replicated cell is faulty. The worst mistake is that they continue to replicate themselves without any limit. In addition, they are often unspecialised or undifferentiated, so they don't serve a purpose, and they do not go through normal cell life cycles.
Because the cells keep dividing, they can eventually form a lump, or mass, which affects the part of the body where they live. This body part may then be unable to work properly, or may not be able to work at all.
Benign tumours are non-cancerous, and do not spread outside their original boundary, which means that they do not affect other tissues. For this reason, they may not be harmful. However, some benign tumours change and become malignant, or cancerous. These tumours can then invade surrounding tissues and harm more of the body. Cancer cells are also able to influence nearby healthy cells to form blood vessels which bring nutrients and oxygen to the tumour, and remove waste products, which helps it to grow.
When cancer cells are located in just one area, it is called a cancer in situ. Many cancers start this way, and form a lump which may be able to be removed surgically. Unfortunately, cancer calls do not always remain isolated, and cells can break off from the mass and travel to other parts of the body, where they form new cancers; this process is called metastasis.
Other cancers form in parts of the body like the lymph nodes and blood system. Because these are the body's highways, moving cells around the body, these cancers are not localised, and spread quickly and easily. When cells from solid tumours break off and get into the blood or lymphatic systems, they can spread throughout the body as well.
Because of the number of new cells made every day, there are inevitably going to be mistakes from time to time. Normally, when a cell is damaged or faulty, it is recognised as wrong by the immune system and destroyed. But sometimes the mistakes, or mutations, that made the cell cancerous mean that the immune system can't detect them, which is why they are able to grow without restriction, and spread throughout the body. This is when those cells can become cancers.
As cancer can form in nearly any type of cell, each of which is separate and unique, cancer is not a single disease, but a group of over 100 different diseases. Because of this, medical specialists will treat different cancers differently, and there are different success rates.
Different types of cancer also grow at different rates. Some cancers are so slow growing that doctors may decide not to treat them, because the treatment would cause more harm than the cancer. Other cancers are so fast growing, or have travelled to so many different parts of the body, that it would be impossible to remove or destroy all of it. This is why it's important to be aware of changes in your body, so that if you do develop cancerous cells, they are found earlier, making treatment easier and more effective.