Cancer is not just one disease, but a group of more than 100 diseases. Its two main characteristics are uncontrolled growth of the cells in the human body and the ability of these cells to migrate from the original site to distant sites. If the spread is not controlled, cancer can result in death.


The American Cancer Society (ACS) estimated that more than 1.7 million Americans would receive a cancer diagnosis in 2018. (This total includes melanomas but does not include basal and squamous cell skin cancers.) The ACS also estimated that more than 609,000 people in the United States would die of cancer in 2018. Cancer is second only to heart disease as the most frequent cause of death overall in the United States.

The five-year relative survival rate for individuals diagnosed with cancer has increased substantially over several decades. In 1975–1977, the survival rate was 49%. This compares with 69% for 2007–2013, according to the ACS. The society credits the increase to improved ability to diagnose certain cancers at an earlier stage (the stage is a description of the spread of the disease at the time of diagnosis: stage I is the earliest stage, stage IV is the most advanced) and to improvements in treatment.

Since the occurrence of cancer increases as individuals age, most cases are seen in adults, middle-aged or older. Among males, the most common cancers are prostate (19%); lung and bronchus (14%); and colon and rectum (9%). Among women, the most common cancers are breast (30%); lung and bronchus (12%); and colon and rectum cancers (8%). Among both men and women, lung and bronchus cancer caused the most deaths in 2017: 84,590 for men, and 71,280 for women. In less advanced and poorer countries around the world, cancer death rates are much higher.

Although most cancer occurs in adults, cancer also occurs in children. In 2017, the American Cancer Society estimated 10,270 new cases to be diagnosed in infants to children 14 years old. About 1,190 children aged under 14 years died of cancer in the United States in 2017. In general, children respond better to cancer treatment than adults do. Advances in treatment have resulted in better outcomes and increased long-term survival rates for children. Nonetheless, new cases of childhood cancer have increased, and cancer was the second-leading cause of death in children in 2017, behind accidents.

Cancer clusters

Cancer clusters are groups of reported cancers that are higher than would be expected for a population. Clusters can occur among families, friends, coworkers, or by geography, such as in a neighborhood. Once investigated, many turn out not to be clusters but are coincidental in nature.


Cancer, by definition, is a disease of the genes. A gene is a small part of DNA, which is the master molecule of the cell. Genes make proteins, which are the ultimate workhorses of the cells. These proteins allow the body to carry out all the many processes that permit an individual to function—to breathe, to think, and to move.

Throughout people's lives, the cells in their bodies are growing, dividing, and replacing themselves.

Common pathogens and associated cancers

Common pathogens and associated cancers

Many genes produce proteins that are involved in controlling the processes of cell growth and division. Certain alterations (mutations) to the DNA molecule can disrupt the genes and produce faulty proteins that can remove a cell's restraints on growth. This abnormal cell begins to divide uncontrollably and eventually forms a new growth called a tumor or neoplasm (medical term for cancer, meaning new growth).

In a healthy individual, the immune system typically recognizes the neoplastic cells and destroys them before they get a chance to divide. However, some mutant cells may escape immune detection and survive to become tumors or cancers.

Tumors are of two types, benign or malignant. A benign tumor is not cancerous. It typically grows slowly, does not spread or invade surrounding tissue, and, once removed, does not usually recur. A malignant tumor, by contrast, is cancerous. It invades surrounding tissue and spreads to other parts of the body. If the cancer cells have spread to the surrounding tissues, cancer can recur even after the malignant tumor is removed.

Several different types of cancer exist:

Causes and symptoms

Most cancers are caused by changes in the cell's DNA that are related to environmental factors. These factors, called carcinogens, are responsible for causing the initial mutation in the DNA, and they come in many types.

Some cancers have a genetic or inherited basis. In other words, individuals can inherit faulty DNA from a parent. This faulty DNA can make it more likely the person will get cancer. Numerous cancers are linked to both environmental and hereditary factors, but fewer than 10% of all cancers are strictly linked to hereditary factors. Cancers that are known to have a hereditary link are those of the breast, colon, ovaries, and uterus. Certain inherited physiological traits may also contribute to cancers. For example, inheriting fair skin makes a person more likely to develop skin cancer, but only if that person also has prolonged exposure to intensive sunlight. This is one of many examples of inherited and environmental factors both affecting a person's risk of developing cancer. Scientists continue to discover links between genes and cancer. Some genes are designed to stop or slow cell division but are somehow altered so they do not stop cancer cells. Others are designed to help cells grow but become faulty genes that stay activated, causing unusual and rapid cell growth.

Major risk factors for cancer are related to tobacco and alcohol use, dietary factors, sexual and reproductive behavior, exposure to infectious agents, family history, occupation, and environmental factors, including pollution.

Cancer is a progressive disease and goes through several stages. Each stage can produce many symptoms. Some symptoms are produced early and may occur because a tumor that is growing within an organ or a gland. As the tumor grows, it can press on the nearby nerves, organs, and blood vessels. This pressure can cause pain, which may be the earliest warning signs of cancer.

More than 100 different types of cancers exist, and each can produce very different symptoms, but the American Cancer Society lists the following as possible warning signals of cancer:

Many other diseases can produce similar symptoms. However, individuals who have any of these symptoms should be examined by a physician as soon as possible, especially if the symptoms linger. The earlier a cancer is diagnosed and treated, the better the chance of survival and/or cure. Many cancers such as breast cancer may not have any early symptom. Routine screening tests such as mammograms and colonoscopies are, therefore, critical in identifying these silent diseases. According to the ACS (2015), breast exams, either from a healthcare professional or self-exams, are no longer recommended.


Diagnosis of many cancers begins with a thorough physical examination and a complete medical history. The doctor will observe, feel, and palpate (apply pressure by touch) different parts of the body to identify variations from the normal size, feel, and texture of the organ or tissue. For instance, the doctor may palpate the lymph nodes in the neck, under the arms, and in the groin, because many illnesses and cancers cause swelling of the lymph nodes.

Mild, nonmalignant. Recovery is favorable with treatment.
The surgical removal and microscopic examination of living tissue for diagnostic purposes.
Bone marrow—
Spongy material that fills the inner cavities of the bones. The progenitors of all the blood cells are produced in this bone marrow.
Any substance capable of causing cancer by mutating the cell's DNA.
Treatment with certain anticancer drugs.
The layer of cells covering the body's surface and lining the internal organs and various glands.
Hormone therapy—
Treatment of cancer by inhibiting the production of hormones such as testosterone and estrogen.
Treatment of cancer by stimulating the body's immune defense system.
A general term for cells and the tumors they form that can invade and destroy other tissues and organs.
The spread of cancer from one part of the body to another.
Describes masses of tissue resulting from cells that divide uncontrollably or do not die as they should.
Radiation therapy—
Treatment using high-energy radiation from x-ray machines, cobalt, radium, or other sources.
An open wound, bruise, or lesion on the skin.
An abnormal growth resulting from a cell that lost its normal growth control restraints and started multiplying uncontrollably.
X rays—
High-energy radiation used in high doses, either to diagnose or treat disease.

The most definitive diagnostic test for cancer is the biopsy, wherein a piece of tissue is surgically removed for examination with a microscope and other possible laboratory studies. Besides confirming a cancer, the biopsy provides information about the type of cancer, the aggressiveness (pace of growth) of the cancer, and the extent of its spread. Since a biopsy provides the most accurate analysis, it is considered the best diagnostic test.


Treatment and prevention of cancers continued to be the focus of a great deal of research as of 2018. Research into new cancer therapies includes cancer-targeting gene therapy, immunotherapy, and other targeted therapies such as monoclonal antibodies. Many therapies require years of clinical testing and research before they can be used.

The aim of cancer treatment is to remove all or as much of the tumor as possible and to prevent the recurrence or spread of the primary tumor. While doctors devise a treatment plan for cancer cases, they realize the likelihood of curing the cancer has to be weighed against the side effects of the treatment. If the cancer is very aggressive and a cure is not possible, then the treatment may be aimed at relieving the symptoms and controlling the cancer for as long as possible.

Cancer treatment can take many different forms and is typically tailored to the individual patient. The decision as to which type of treatment is the most appropriate depends on the type and location of cancer; the extent to which it has already spread; the patient's age, sex, general health status, and personal treatment preferences. The major types of treatment are: surgery, radiation, chemotherapy, biological therapy, targeted therapy, hormone therapy, and bone-marrow and stem cell transplantation.


Preventive surgery is an option in some cases. Preventive or prophylactic surgery involves removal of tissue that is likely to become malignant over time. For example, women who have cancer in one breast and who are at high risk of developing breast cancer in the other breast due to a genetic predisposition for the disease may opt to have a double mastectomy rather than wait and worry about getting the disease in the second breast.

Radiation therapy

Radiation kills tumor cells. Radiation is used alone in cases in which a tumor is unsuitable for surgery. More often, it is used in conjunction with surgery and chemotherapy. Radiation can be either external or internal. In the external form, the radiation is aimed at the tumor from outside the body. Some techniques allow doctors to target therapy precisely to cancerous cells, even accounting for body movement from breathing. In internal radiation (also known as brachytherapy), a radioactive substance in the form of pellets or liquid is placed at the cancerous site by means of a pill, injection, or insertion in a sealed container.


Chemotherapy is the use of drugs to kill cancer cells. It destroys the hard-to-detect cancer cells that have spread and are circulating in the body. Chemo-therapeutic drugs can be given in many forms. The most common administration methods are oral (by mouth) or intravenous administration. Chemotherapy may be given alone or in conjunction with surgery, radiation, or both.

When chemotherapy is used before surgery or radiation, it is known as primary chemotherapy or neoadjuvant chemotherapy. An advantage of neoadjuvant chemotherapy is that since the cancer cells have not been exposed to anti-cancer drugs, they are especially vulnerable. It can, therefore, be used effectively to reduce the size of the tumor for surgery or target it for radiation. The more common use of chemotherapy is adjuvant therapy, which is given to enhance the effectiveness of other treatments. For example, after surgery, adjuvant chemotherapy may be given to destroy cancerous cells that remain in the body.

Biological and targeted therapies

Biological and targeted therapies use the body's own immune system to destroy cancer cells. These include:

Hormone therapy

Hormone therapy is standard treatment for some types of cancers that are hormone-dependent and grow faster in the presence of particular hormones. These include cancer of the prostate, breast, and uterus. Hormone therapy may involve blocking the production or action of these hormones or surgically removing the hormone-producing gland. These actions can slow the growth of the tumor and, in some cases, can halt the progression of the disease for many years.

Bone marrow, stem cell, and cord-blood transplantation

Bone marrow is the tissue within the bone cavities that contains blood-forming cells. Healthy bone marrow tissue constantly replenishes the blood supply and is essential to life.

A bone marrow transplant is the removal of marrow from one person and the transplant of the blood-forming cells either to the same person or to someone else. Bone-marrow transplantation, while not a therapy in itself, is often used to rescue patients, by allowing those with cancer to undergo aggressive therapy. Stem cell transplants have been performed to replace bone marrow that has been destroyed by cancer, chemotherapy, or radiation therapy. Stem cells are specialized cells in the bone marrow from which the body receives a constant source of blood cells. Stem cells may also be harvested from umbilical cords, a process that is referred to as a cord blood transplant. Cancers in which stem cell transplants may be used include leukemia, lymphoma, and multiple myeloma.


Many cancers are curable if detected and treated in their early stages. A cancer patient's prognosis is affected by many factors, particularly the type of cancer the patient has, the stage of the cancer, the extent to which it has metastasized, and the aggressiveness of the cancer. In addition, the patient's age, general health status, and the effectiveness of the treatment being pursued are important factors.

Recommendations for cancer screening

Recommendations for cancer screening
SOURCE: American Cancer Society.


Screening examinations conducted regularly by healthcare professionals can result in the detection of a wide variety of cancers at early stages, when treatment is more likely to be successful. These include breast, colon, rectum, cervix, prostate, testis, tongue, mouth, and skin cancer. Routine screening test recommendations also change often based on research, and individuals should discuss screening with their doctors. Examples of recommendations are:

As of 2018, the American Cancer Society recommends that men talk to their doctor about whether to be tested for prostate cancer beginning at age 50 or younger for men at high risk for the disease.

A revolution in molecular biology and cancer genetics has contributed a great deal to the development of several tests designed to help predict whether a person will develop cancer. These new techniques include genetic testing, in which molecular probes are used to identify mutations in certain genes that have been linked to particular cancers. The American Society of Clinical Oncology recommends that physicians offer genetic testing when their patients have a family history that suggests a genetic cause of cancer. The timing and justification of genetic testing vary depending on the suspected risk and type of cancer in a family. Genetic testing also can be used to make decisions regarding treatment after a cancer is diagnosed.

In addition to new and better genetic tests, researchers are trying more simple or accurate methods to detect cancer for use in widespread screening. One effort is to develop ways to use breath testing to spot compounds present in the breath of people who have cancer.

Individuals can also take steps on their own to reduce their risks of developing cancer. These include:

Public health role and response

It is a very rare individual who has not been touched by cancer either through a personal illness or the illness of a family member or friend. Global population booms, aging of populations, and lack of money or medical resources in some countries contribute to cancer incidence and deaths. Because cancer is such a widespread disease, the medical profession, as well as government and other public and private agencies, have placed a major emphasis on the importance of cancer prevention, early diagnosis, new treatment options, and cancer research.


In 2018, the National Institutes of Health (NIH) estimated that overall costs of treating cancer in the United States would be $158 billion in 2020, including medical care and other costs, such as those related to lost productivity due to illness, care, and premature death.

Governments, as well as public and private organizations, have mounted numerous campaigns to help prevent cancer. These include campaigns warning of the increased risk of lung cancer associated with tobacco use, encouraging the use of sunscreen to prevent skin cancer, and promoting the importance of a diet and exercise to stave off many types of cancer and other diseases. At the same time, treatments and preventive measures are improving and evolving as researchers discover more about the different types of cancer, why they begin, how they develop, and how they spread.

See also Leukemia ; Melanoma ; Radiation ; Radiation exposure ; Smoking .



Bollinger, Ty M. The Truth about Cancer: What You Need to Know about Cancer's History, Treatment, and Prevention. Carlsbad, CA: Hay House, 2016.

Servan-Schreiber, David. Anticancer: A New Way of Life. New York: Penguin Books, 2017.


Hoeve, John E. Ten, and Mark Z. Jacobson. “Worldwide Health Effects of the Fukushima Daiichi Nuclear Accident.” Energy & Environmental Science 5, no. 9 (2012): 8743–57.

Kabir, K. M. Mohibul, and William A. Donald. “Worldwide Cancer Breath Testing: A Patient Review.” Expert Opinion on Therapeutic Patients 28, no. 3 (January 2018): 227–39.


American Cancer Society. “American Cancer Society Guidelines for the Early Detection of Cancer.” (accessed March 19, 2018).

American Cancer Society. “Cancer Facts & Figures 2018.” (accessed March 19, 2018).

American Cancer Society. “Stem Cell Transplant for Cancer.” (accessed March 19, 2018).

American Society of Clinical Oncology. “Clinical Cancer Advances 2018.” (accessed March 19, 2018).

American Society of Clinical Oncology. “Genetic Testing for Cancer Risk.” (accessed March 19, 2018).

National Cancer Institute. “Cancer Clusters.” U.S. National Institutes of Health. (accessed March 20, 2018).

National Cancer Institute. “HPV and Cancer.” U.S. National Institutes of Health. (accessed March 19, 2018).

National Cancer Institute. “Infectious Agents and Cancer.” U.S. National Institutes of Health. (accessed March 19, 2018).

World Health Organization. “Cancer.” (accessed March 19, 2018).


American Cancer Society, 250 Williams St. NW, Atlanta, GA, 30303, (800) 227-2345, .

National Cancer Institute, 6116 Executive Blvd., Ste. 300, Bethesda, MD, 20892-8322, (800) 422-6237, .

National Coalition for Cancer Survivorship, 8455 Colesville Rd., Silver Spring, MD, 20910, (877) 622-7937, .

Rosalyn Carson-DeWitt, MD
Revised by Teresa Odle, BA, ELS

  This information is not a tool for self-diagnosis or a substitute for professional care.