What Is Cancer?

Table of Contents
View All

Cancer is a group of over 200 different diseases in which abnormal cells multiply uncontrollably, destroying body tissues or functions. The most common types are basal cell skin cancer, breast cancer, prostate cancer, lung cancer, colon cancer, melanoma, leukemia, and lymphoma. Cancer has various symptoms and is treated in different ways depending on the type. Treatments include surgery, chemotherapy, radiation, and targeted therapies.

One in two men and one in three women are expected to develop cancer (not including skin cancer) over the course of a lifetime.

Types of Cancer

Each cancer is named for the tissue type or organ in which it begins. Some of these are very common (for example, one in seven men is expected to develop prostate cancer). Some are very rare, occurring in only a few people each year.

The top 10 most common cancers to occur other than skin cancer (incidence of new cases in men and women combined) include:

  1. Breast cancer
  2. Lung cancer
  3. Prostate cancer
  4. Colon cancer
  5. Bladder cancer
  6. Melanoma
  7. Non-Hodgkin lymphoma
  8. Thyroid Cancer
  9. Kidney cancer
  10. Leukemia

For men, oral cavity and pharynx cancers, are among the top 10 in terms of occurrence. and for women, uterine cancer, is in the top

10 most common cancers for occurrence. For both men and women, pancreatic

cancer are both in the top 10 most common cancers for occurrence and are in the

top 10 most fatal cancers.

Less common cancers that still affect a significant number of people each year in the United States include ovarian cancer, testicular cancer, brain tumors. and myeloma. There are many types of uncommon and rare cancers.

Cancer Symptoms

Having an awareness of the most common symptoms of cancer, and talking to your doctor if you experience any of them, is critical in taking good care of your health.

Common symptoms of cancer include:

Less common symptoms occur with many cancers but are no less important. Examples include jaundice, a yellowish discoloration of the skin, and even new onset depression.

Talk to your doctor if you have any symptoms that are unexplained. If those symptoms remain as such after you see your doctor, it's important to go back for another visit or get a second opinion.

Many cancer survivors are alive because they were their own advocates and did not settle for an unconfirmed diagnosis.


For a normal cell to become a cancer cell, a series of gene mutations needs to take place. The mechanisms leading to these mutations include:

  • Direct damage to DNA: Some substances and exposures can directly damage the DNA in your cells. An example would be radiation or some carcinogens from the environment.
  • Chronic inflammation: Whenever cells reproduce and divide, there is a chance that an accident (a DNA mutation) will happen. Chronic inflammation, such as that in the respiratory tree or esophagus related to smoking, may result in cancer by increasing the chance that such a mistake in cell division will occur.

There are several risk factors that account for cells becoming cancer cells. These can be broken down to include:

  • Lifestyle factors: Smoking is the single most important cause of cancer. Smoking causes many different types of cancer and is responsible for nearly a third of cancer deaths. Obesity may actually soon surpass smoking as the leading preventable cause of cancer in the U.S., and unfortunately, it's thought that only a third of Americans are aware of this risk. Excess alcohol is also associated with an increased risk of several cancers.
  • Environmental exposures: Radon exposure causes cancer and having an elevated level of this gas in your home is the second most common cause of lung cancer. Work-related exposures to cancer-causing substances (carcinogens) is an important cause of cancer in men, but increasingly, in women as well.
  • Genetics: A genetic predisposition to cancer can occur when people inherit a mutation in a gene responsible for removing damaged cells (tumor suppressor genes) and more.
  • Viruses and other microorganisms: Viruses are an important cause of cancer, responsible for roughly 25% of cancers worldwide, and 5% to 10% of cancers in the U.S. Infection with human papillomavirus (HPV), for example, is the cause of most cervical cancers, as well as some other types. H. pylori is thought to be the cause of many stomach cancers in the United States.

It's important to note that a cancer can be primary or secondary. For example, cancer that originates in the lung is primary lung cancer. Cancer that occurs in the lung because of cancer cells that have spread from, say, the breast is considered secondary lung cancer (a.k.a., cancer metastatic to the lung). The cancer cells in these two cases are not the same.


Screening tests are done routinely for some types of cancer, such as fecal occult blood for colon cancer, PSA test for prostate cancer, mammograms for breast cancer, and the PAP test for cervical cancer. Or, you may go to your doctor because you are having some of the symptoms of cancer.

The next diagnostic steps will depend on the type of cancer. You can expect blood tests and urine tests looking at your overall health as well as high or low amounts of some substances or, in the case of blood cancers, abnormal cells.

Imaging studies may be done to look for tumors. These include X-rays, CT scans, ultrasound, MRI, PET scans, and nuclear scans.

When a tumor is suspected, your doctor will usually do a biopsy so cells can be analyzed to determine whether the growth is cancerous.

The diagnostic process may also include staging, which determines whether the cancer is localized or has spread, and helps guide treatment. Further testing may be done, including tumor markers and genetic tests, which help determine the exact type of cancer.


The best treatments for cancer depend on the type and stage of the cancer and many other factors. Treatments for cancer can be separated into two main categories:

  • Local treatments include surgery and radiation therapy. These treat a cancer where it began but are unable to reach cancer cells that may have traveled away from the primary cancer via the bloodstream or lymphatic system. When cancers are caught early, local treatments can often cure the cancer.
  • Systemic treatments include chemotherapy, targeted therapies, hormonal therapies, and immunotherapy, and treat cancer cells wherever they happen to be in the body. Systemic therapies are usually needed if a cancer has spread (or if there is a chance it has spread) and for blood-related cancers.

Every cancer is different on a molecular level, so two people with the same type and stage of cancer could have cancers that respond to treatments in very different ways.

Cancer treatment options may include:

  • Surgery: For solid tumors, surgery often offers the best chance to cure a cancer.
  • Chemotherapy: Chemotherapy refers to the use of cytotoxic chemicals to kill cancer cells.
  • Radiation therapy: Radiation therapy uses high-energy radiation to kill cancer cells.
  • Targeted therapies: Targeted therapies target cancer cells or processes important to cancer cells specifically.
  • Immunotherapy: Immunotherapy is an exciting new treatment approach to cancer, taking into account that our immune systems often know how to fight cancer.
  • Hormonal therapies: With some cancers, hormones produced naturally in the body can bind to and stimulate the growth of cancer cells. This may be prevented by decreasing the production of hormones in the body or by blocking the ability of the hormones to have these effects on cancer cells.
  • Stem cell transplant: Stem cell transplants may be used following high-dose chemotherapy or radiation therapy to replace the blood cells in the bone marrow.


A cancer diagnosis is often shocking, and you can expect to go through a range of emotions. You may be angry, numb, confused, sad, or anxious. These reactions are normal and can change from hour to hour, day to day. Your friends and family may also go through this emotional turmoil.

If you've been diagnosed with cancer, reach out to your loved ones. Don't try to go it alone. Learn to let others help you. Ask a lot of questions and be your own advocate in your cancer treatment.

Learning the diagnosis is just the start of your journey through cancer. Treatments for cancer can be grueling, and a significant number of people have some ongoing symptoms related to treatment long after that treatment is done. You will want to know practical ways to deal with these symptoms as well as manage the financial impact.

Rehabilitation—whether it is to restore physical function lost due to cancer, coping with the post-traumatic stress common to survivors, or to decrease the disability from lymphedema, can make a difference in the quality of life for people who have survived cancer.

Whether you are newly diagnosed and looking for support, or have completed treatment and entered survivorship, there are many organizations through which you can receive support from others.

For friends and family members, the most important thing is your presence, which can bring comfort when the rest of life doesn't seem to be following the rules. Supporting a loved one with cancer also means taking care of yourself. That is easier said than done, but it can make a big difference as time goes on.

A Word From Verywell

Cancer is a frightening disease and emotions can run deep if you hear this word in the same sentence as your name or that of a loved one. It can help to know that treatments—and survival rates—for cancer are improving steadily. More than half of people with cancer enjoy long-term survival after their initial diagnosis, and there are an estimated 15 million cancer survivors in the United States alone.

At the same time, experts are learning more about the causes and what might be done to prevent cancer in the first place. Research and advances are taking place every day, with hundreds of medications currently being studied in clinical trials.

Was this page helpful?
Article Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.