Cancer is a type of disease where cells grow out of control, divide and invade other tissues.  In a person without cancer, cell division is under control. In most tissues, healthy cells divide in a controlled way and copy themselves to create new healthy cells.  With cancer, this normal cell division goes out of control. Cells change their nature because mutations have occurred in their genes. All the daughter cells of cancer cells are also cancerous.

If the abnormal cells do not invade, but just divide and swell up their original tissue, this is not called "cancer". It is called a tumour. Tumours are usually not a threat to life because they can be cut out. However, some tumours occur in places where they cannot be cut out, and they can be fatal. Some brain tumours are of this type.

The symptoms of cancer are caused by the cancerous cells invading other tissues. This is called metastasis. Metastasis is how which cancer cells move through the bloodstream or lymphatic system. When this happens, a person's cancer can be spread throughout his or her body. Eventually those other tissues cannot work as well, and the whole body begins to get worse, and may die.

Cancer can affect anybody at any age. Most types of cancer are more likely to affect people as they get older. This is because as a person's DNA gets older, their DNA may become damaged, or damage that happened in the past may get worse.  One type of cancer that is more common in young men, rather than older people, is testicular cancer (cancer of the testicles).

Cancer is one of the biggest and most researched causes of death in developed countries. Studying cancer and its treatment is called oncology.

Causes 
Cancer is one of the most common causes of death around the world. It causes about 12.5% (or one out of every eight) of all deaths worldwide, according to the World Health Organization.

Different types of cancer have different causes. Some things are known to cause cancer in a specific body part; other things are known to be able to cause many different types of cancer. For example, using tobacco (smoked or smokeless) can cause many types of cancers, such as lung, mouth, tongue, and throat cancers. Other things that are known to be able to cause cancer - or make a person more likely to get cancer - include: radiation including sunlight and X-rays in large or many doses, and exposure to radiation (for example in a nuclear power plant); chemicals and materials used in building and manufacturing (for example, asbestos and benzene); high-fat or low-fiber diets; air and water pollution; eating very little fruits and vegetables; obesity; not enough physical activity; drinking too much alcohol; and certain chemicals commonly used at home. Some cancers can also be caused by viruses.  Many people who are exposed to these things do get cancer - but some do not.

Kinds 
There are many different kinds of cancers. Some of the most common are: 
 Breast cancer 
 Brain cancer 
 Leukemia (a blood cancer) 
 Testicular cancer  
 Mesothelioma (which starts in the lungs, and is usually caused by breathing in asbestos for long periods of time) 
 Lung cancer

Treatment of cancer 
There is no sure cure for cancer. It can only be cured if all of the cancerous cells are cut out or killed in place. This means that the earlier the cancer is treated, the better the chances are for a cure (because the cancer cells may not have had enough time to copy themselves and spread so much that the person cannot be cured).

There are a few different types of treatments that may kill cancer cells.  These treatments are:
 Radiotherapy (radiation therapy), which uses radiation to kill cancer cells
 Chemotherapy, which uses strong medications to kill cancer cells
 Immunotherapy works by "inducing, enhancing, or suppressing an immune response".
 Surgery to take out part or all of a tumor
 After surgery, many patients may need radiotherapy or chemotherapy to keep the tumor from growing again

Treating cancer is complicated 
There are a few reasons why treating cancer is complicated.  For example:
 Most things that kill cancer cells also kill normal, healthy cells.  This can cause many side effects, like hair loss and vomiting.
 The body's immune system usually will not attack cancer cells, even though they could easily kill the body. This is because the cancer has actually become a part of the body by invading cells and tissues.  So the immune system sees the cancer as part of the body it is trying to protect, not as a threat to be attacked. 
 There are many different types of cancer, and each has its own symptoms and causes. Even with the same type of cancer, different people may have different symptoms, and may react to treatments differently; their cancer also may grow or spread at different speeds. Treatment has to be a good fit to both the type of cancer and the individual patient who has the cancer.

Many, many people in many countries study cancer and work on finding treatments. There has been some good progress in finding treatments, and many cancers are treated with success. Along with looking for different medical treatments to treat cancer, some studies also look for things that people with cancer can do themselves to try to make themselves healthier. For example, one study showed that if a person with lymphedema (a swelling of the arm linked to breast cancer) lifts weights, he may be able to fight his cancer better than somebody who does not lift weights.

History 
Cancer has been around for many thousands of years.  Today, a lot of the medical terms used to describe cancer come from ancient Greek and Latin.  For example, the Latinized Greek word carcinoma is used to describe a malignant tumor - a tumor made up of cancer cells. The Greeks also used the word "karkinos", which would be translated by Aulus Cornelius Celsus into the Latin word cancer. The prefix 'carcino' is still used in medical words like carcinoma and carcinogenic. A famous Greek doctor, Galen, helped create another word that is very important to medicine today by using the word "onkos" to describe all tumours. This is where the word oncology, the branch of medicine that deals with cancer, comes from.

Ancient history 
Hippocrates (a very famous ancient doctor who is often called the father of modern medicine) named many kinds of cancer. He called benign tumours (tumors that are not made up of cancer cells) oncos. In Greek, onkos means 'swelling'. He called malignant tumours karkinos. This means crab or crayfish in Greek. He used this term because he thought that if a solid malignant tumor was cut into, its veins looked like a crab: "the veins stretched on all sides as the animal the crab has its feet, whence it derives (gets) its name". Hippocrates later added -oma (Greek for 'swelling') after the word 'carcinos'. This is how the word carcinoma came about.

Because the ancient Greeks did not believe in opening up dead bodies to study them, Hippocrates was only able to describe and make drawings of tumors he saw from the outside of the body. He drew tumors that had been on the skin, nose, and breasts.

Hippocrates and other doctors at that time treated people based on the humor theory. This theory said that there were four types of fluid in the body (black, yellow bile, blood, and phlegm). Doctors tried to figure out whether these four "humors" (or body fluids) were in balance. They would then use treatments like blood-letting (cutting the patient and letting him bleed so that he would lose blood); laxatives (giving the patient foods or herbs to make him go to the bathroom), and/or changing the patient's diet. The doctors thought that these treatments would work to get the patient's four humors back into the right balance. The humor theory treatment was popular until the 19th century (the 1800s), when cells were discovered. By this time, people had realized that cancer can happen anywhere in the body.

Early surgery 
The oldest known document that talks about cancer was discovered in Egypt and is thought to be from about 1600 B.C. The document talks about using surgery to treat eight cases of ulcers of the breast. These were treated by cauterization - by burning them - using a tool called "the fire drill". The document also says about cancer, "There is no treatment".

Another very early type of surgery used to treat cancer was written about in the 1020s.  In The Canon of Medicine, Avicenna (Ibn Sina) said that treatment should involve cutting out all diseased tissue.  This included the use of amputation (removing a part of the body completely) or removing veins that ran in the direction of the tumor. Avicenna also suggested that the area that had been treated should be cauterized (or burned) if needed.

The 16th and 17th centuries 
In the 16th and 17th centuries (the 1500s and 1600s), doctors started to be allowed to dissect bodies (or cut them open after death) in order to figure out the cause of death. Around this time, there were many different ideas about what caused cancer. The German professor Wilhelm Fabry believed that breast cancer was caused by a clot of milk in the part of a woman's breast that produces milk. The Dutch professor Francois de la Boe Sylvius believed that all disease was caused by chemical processes. He thought that cancer, in particular, was caused by acidic lymph. Nicolaes Tulp, who lived at the same time as Sylvius, believed that cancer was a poison that slowly spreads and was contagious.

A British surgeon named Percivall Pott was the first person to figure out one of the real causes of cancer. In 1775, he discovered that cancer of the scrotum was a common disease among chimney sweeps (people who cleaned out chimneys). Other doctors started studying this topic and coming up with other ideas about what causes cancer. Doctors then started working together and coming up with better ideas.

The 18th century 
In the 18th century (the 1700s), many people started to use the microscope, and this made a big difference in helping doctors and scientists understand more about cancer. Using the microscope, scientists were able to see that the 'cancer poison' spread from one tumor through the lymph nodes to other sites ("metastasis"). This was first made clear by the English surgeon Campbell De Morgan, between 1871 and 1874.

Before the 19th century (the 1800s), using surgery to treat cancer usually had bad results. Doctors did not understand how important hygiene (or keeping things clean) is for preventing disease, especially after surgery. Because things were not kept clean during or after surgery, patients often got infections and died. For example, one well-known Scottish surgeon, Alexander Monro, kept records and found that 58 patients out of every 60 who had surgery for breast tumors died within the next two years.

The 19th century 
In the 19th century, surgical hygiene got better because of asepsis. Doctors realized that dirtiness and germs cause infections, so they started to keep things cleaner and do things to kill germs in order to prevent their patients from getting infections. It became more common for people to survive after having surgery. Surgical removal of the tumor (taking the tumor out of the body by doing surgery) became the first-choice treatment for cancer. For this kind of treatment to work, the surgeon doing the operation had to be very good at removing tumors. (This meant that even if people had the same kind of cancer, they could get very different results, with some getting good treatment that worked and others getting treatment that did not work, because of differences in how good different surgeons were.)

In the late 1800s, doctors and scientists started to realize that the body is made up of many kinds of tissues, which in turn are made up of millions of cells. The discovery started the age of cellular pathology (studying cells to learn about diseases and figure out what is wrong with the body).

Discovery of radiation 
In the 1890s, French scientists discovered radioactive decay.  Radiation therapy became the first cancer treatment that worked and did not involve surgery.  It required a new multi-disciplinary approach to cancer treatment (people doing different jobs were working together to treat patients).  The surgeon was no longer working by himself - he worked together with hospital radiologists (people who gave and read X-rays) to help patients.  This team approach meant changes in how they worked.  The different people on the team had to communicate with each other and work together, which they were not used to doing.  It also meant that treatment had to be done in a hospital rather than at the patient's home.  Because of this, patients' information had to be put together into files kept at the hospital (called "medical records"). Because this information was now being kept and written down, scientists were able to do the first statistical patient studies using numbers to study questions like how many people who have a certain type of cancer or get a certain treatment survive.

The 20th century 
Another important step forward in understanding cancer happened in 1926, when Janet Lane-Claypon published a paper on cancer epidemiology. (Epidemiology is a field of study which looks at how common a disease is, what patterns the disease takes in different kinds of people, and what this means for understanding and treating the disease.) This historic paper was a comparative study, which tries to find out what causes a disease by looking at a group of people who have the disease and figuring out how they are different from another group that does not have the disease. Lane-Clayton's study looked at 1000 people who all had the same background and lifestyle (or way of living): 500 people with breast cancer and 500 control patients (people without breast cancer). These people were the same in many ways, but some got breast cancer and some did not. To figure out what might be causing certain people to get breast cancer, the study looked at what was different about these people when they were compared to (or looked at alongside) the people who did not get cancer.

Lane-Clayton's study was published by the British Ministry of Health. Her work on cancer epidemiology was continued by Richard Doll and Austin Bradford Hill. They used the same ways of studying cancer as Lane-Clayton, but they looked at a different kind of cancer: lung cancer. In 1956, they published their results in a paper called "Lung Cancer and Other Causes of Death In Relation to Smoking. A Second Report on the Mortality of British Doctors" (also called the British doctors study). Later, Richard Doll left the London Medical Research Center (MRC), and started the Oxford unit for Cancer epidemiology in 1968. By using computers, this unit was able to do something new and very important: it brought together large amounts of cancer data (pieces of information about cancer). This way of studying cancer is very important to cancer epidemiology today, and it has also been very important in shaping what we now know about cancer and what the rules and laws about the disease and public health are today. Over the past 50 years, many different people have done a lot of work to collect data from different doctors, hospitals, areas, states, and even countries. This data is used to study whether different kinds of cancer are more or less common in different areas, environments (for example, in big cities compared to the countryside), or cultures. This helps people who study cancer to figure out what makes people more or less likely to get different kinds of cancer.

Effects of World War II 
Before World War II, doctors and hospitals were getting better at collecting (or getting and keeping) data about their patients who had cancer, but it was rare for this data to be shared with other doctors or hospitals. This changed after WWII, when medical research centers found out that different countries had very different number of cases of cancer. Because of this, many countries created national public health organizations (which studied public health issues in an entire country). These national public health organizations began to bring together health data from many different doctors and hospitals. This helped them figure out some of the reasons why cancer was so much more common in certain places. For example, in Japan, people studying cancer found out that people who had survived the atomic bombings of Hiroshima and Nagasaki had bone marrow that was completely destroyed. This helped them realize that diseased bone marrow could also be destroyed with radiation, which was a very important step in figuring out that leukemia (a blood cancer) can be treated with bone marrow transplants.

Since World War II, scientists have kept finding better cancer treatments. However, there are some things that still need to get better. For example, while there are good treatments for many kinds of cancer, there are still no treatments for certain kinds of cancer, or for some cancers once they progress (or get worse) to a certain stage of the disease. Also, the cancer treatments that do exist are not all standardized (there is not one agreed-upon way of giving every treatment which is used each time the treatment is given). Cancer treatments are also not available everywhere in the world.  People need to keep studying cancer epidemiology and forming international partnerships (where different countries work together) to find cures and make cancer treatments available everywhere.

Other websites 
 What is Cancer?  (Simple English) - American Cancer Society

References