Cancer: Symptoms, Causes, Stages, and Treatment in One Complete Guide
Updated: 2 days ago
Medically reviewed by Dr. Baraa Alnahhal, MD · Last reviewed: September 2026
Editorial note: This article is for educational purposes only. It is not medical advice and cannot replace professional diagnosis or treatment. If you have symptoms that concern you, see a healthcare professional. Last updated: August 2026.
TL;DR
Cancer is not one disease but a family of diseases caused by gene mutations that make cells divide uncontrollably and invade healthy tissue. It is the second-leading cause of death worldwide, yet survival rates are improving. Early warning signs include fatigue, unexplained weight changes, lumps, and persistent symptoms that do not go away. Most cancers are diagnosed through a biopsy, staged from 0 to IV, and treated with surgery, chemotherapy, radiation, immunotherapy, hormone therapy, or targeted drugs — often in combination.
Quick Answer
Cancer is caused by mutations in a cell's DNA that remove the normal controls on growth, letting abnormal cells multiply and spread. Most diagnosed patients are 65 or older, and the majority of mutations happen after birth due to factors like smoking, radiation, viruses, and obesity rather than inheritance. Doctors confirm cancer with a biopsy, stage it from 0 to IV, and treat it with surgery, chemotherapy, radiation, immunotherapy, hormone therapy, or targeted drugs. Many cancers are preventable through smoking cessation, sun protection, healthy weight, exercise, and recommended vaccinations and screenings.
What is cancer?
Cancer refers to any one of a large number of diseases characterized by the development of abnormal cells that divide uncontrollably and have the ability to infiltrate and destroy normal body tissue [1]. Cancer often has the ability to spread throughout the body.
Cancer is the second-leading cause of death in the world. But survival rates are improving for many types of cancer, thanks to improvements in cancer screening, treatment, and prevention [1].
The scale is large. In 2026 alone, approximately 2.1 million new cancer cases and 626,140 cancer deaths are projected in the United States [3]. Worldwide, roughly 20 million new cases and 9.8 million deaths occur each year [5].
How common is cancer, and which types are most frequent?
Four cancers — breast, prostate, lung, and colorectal — account for nearly half of all new cancer cases in the United States [4] [6].
Cancer type | Where it stands (U.S.) | Global note |
Breast | Most common on the NCI list; about 319,750 new cases expected in 2025 | Among the most diagnosed worldwide, especially in women |
Prostate | One of the four cancers making up half of U.S. cases | Among the top diagnoses in men globally |
Lung | Among the top four in the U.S. | The leading cause of cancer death worldwide, with about 1.8–1.9 million deaths per year [5] [7] |
Colorectal | Among the top four in the U.S. | Second-leading cause of cancer death globally (~900,000 deaths) [5] |
How does cancer start?
Cancer is caused by changes (mutations) to the DNA within cells. The DNA inside a cell is packaged into a large number of individual genes, each of which contains a set of instructions telling the cell what functions to perform, as well as how to grow and divide. Errors in the instructions can cause the cell to stop its normal function and may allow a cell to become cancerous [1].
What do gene mutations do?
A gene mutation can instruct a healthy cell to do one of three things [1]:
Mutation effect | What happens |
Allow rapid growth | The cell grows and divides more rapidly, creating many new cells that all carry the same mutation |
Fail to stop uncontrolled growth | Cancer cells lose the controls (tumor suppressor genes) that tell normal cells when to stop growing |
Make mistakes when repairing DNA errors | DNA repair genes normally find and fix errors; a mutation in these genes lets other errors go uncorrected, leading cells toward cancer |
These are the most common mutations found in cancer, but many others can contribute [1].
What causes gene mutations?
Gene mutations can arise in two broad ways [1]:
Type | How it happens | Share of cancers |
Inherited (present at birth) | A mutation passed down from a parent | A small percentage of all cancers [9] |
Acquired (after birth) | Most mutations occur after birth and are not inherited; driven by smoking, radiation, viruses, cancer-causing chemicals (carcinogens), obesity, hormones, chronic inflammation, and a lack of exercise | The large majority |
Gene mutations occur frequently during normal cell growth, and cells contain a repair mechanism to fix most mistakes. Occasionally a mistake is missed — and that missed error can allow a cell to become cancerous [1].
How do mutations interact?
The mutations you are born with and those you acquire during life work together to cause cancer. Inheriting a mutation that predisposes you to cancer does not guarantee you will develop it — you may need one or more additional mutations to trigger the disease. The inherited mutation can, however, make you more likely than other people to develop cancer when exposed to a certain cancer-causing substance [1].
It is not clear how many mutations must accumulate for cancer to form, and the number likely varies among cancer types [1].
What are the symptoms of cancer?
Signs and symptoms vary depending on what part of the body is affected. Some general signs are associated with, but not specific to, cancer [1]:
Symptom | What to watch for |
Fatigue | Persistent tiredness that rest does not fix |
Lump or thickening | An area that can be felt under the skin |
Weight changes | Unintended loss or gain |
Skin changes | Yellowing, darkening, or redness; sores that will not heal; changes to existing moles |
Bowel or bladder habit changes | Persistent changes in how you go |
Cough or breathing trouble | Persistent cough or trouble breathing |
Swallowing trouble | Difficulty swallowing |
Hoarseness | Voice changes that do not clear up |
Indigestion | Persistent indigestion or discomfort after eating |
Muscle or joint pain | Persistent, unexplained pain |
Fevers or night sweats | Persistent, unexplained episodes |
Bleeding or bruising | Unexplained bleeding or bruising |
When to see a doctor. Make an appointment if you have any persistent signs or symptoms that concern you. Even without symptoms, if you are worried about your cancer risk, ask your doctor which screening tests are appropriate for you [1].
Who is at higher risk of cancer?
While doctors have an idea of what may increase cancer risk, the majority of cancers occur in people who do not have any known risk factors [1]. Known risk factors include:
Risk factor | Why it matters |
Your age | Cancer can take decades to develop; most people diagnosed with cancer are 65 or older — though it can be diagnosed at any age |
Your habits | Smoking, drinking more than one drink a day for women (up to two for men), excessive sun exposure or frequent blistering sunburns, obesity, and unsafe sex all contribute |
Your family history | Only a small portion of cancers are due to an inherited condition; genetic testing may be an option if cancer runs in your family — and having a mutation does not mean you will get cancer |
Your health conditions | Chronic conditions such as ulcerative colitis can markedly increase the risk of certain cancers |
Your environment | Secondhand smoke, asbestos, benzene, and other chemicals at home or work raise risk [1] |
What complications can cancer cause?
Cancer and its treatment can cause several complications [1]:
Complication | Notes |
Pain | Caused by cancer or its treatment; not all cancer is painful, and pain can often be treated effectively |
Fatigue | Common, especially with chemotherapy or radiation, and usually temporary |
Difficulty breathing | Cancer or treatment may cause shortness of breath; treatments may bring relief |
Nausea | Sometimes predictable in advance; medications can prevent or reduce it |
Diarrhea or constipation | Cancer and treatment can affect the bowels |
Weight loss | Cancer steals food from normal cells; tube feeding usually does not reverse this type of weight loss |
Chemical imbalances | Excessive thirst, frequent urination, constipation, and confusion can signal them |
Brain and nervous system problems | Pressure on nerves; brain involvement can cause headaches and stroke-like signs such as one-sided weakness |
Paraneoplastic syndromes | Rare immune reactions attacking healthy cells, causing symptoms such as difficulty walking or seizures |
Metastasis | As cancer advances, it may spread to other parts of the body depending on the type |
Recurrence | Survivors carry a recurrence risk; follow-up plans with periodic scans and exams help detect it early |
Can cancer be prevented?
Doctors have identified several ways to reduce cancer risk [1]:
Prevention step | What it involves |
Stop smoking | Quitting now reduces future risk; smoking is linked to several cancers, not just lung cancer |
Avoid excessive sun | Limit exposure with shade, protective clothing, and sunscreen to lower skin cancer risk |
Eat a healthy diet | Rich in fruits and vegetables, whole grains, and lean proteins; limit processed meats |
Exercise | Aim for at least 30 minutes most days of the week |
Maintain a healthy weight | Being overweight or obese may increase risk |
Moderate alcohol | Up to one drink a day for women, up to two for men |
Schedule cancer screening exams | Based on your personal risk factors |
Ask about immunizations | Hepatitis B vaccination reduces liver cancer risk; HPV vaccination reduces cervical and other cancer risk |
How is cancer diagnosed?
Diagnosing cancer at its earliest stages often provides the best chance for a cure, so discuss appropriate screening with your doctor. For some cancers, screening tests have been shown to save lives; for others, screening is recommended only for people at increased risk [2].
Doctors may use one or more approaches to diagnose cancer [2]:
Approach | What it involves |
Physical exam | Feeling for lumps; looking for skin color changes or organ enlargement |
Laboratory tests | Blood and urine tests; for example, a complete blood count may reveal unusual white blood cells in leukemia |
Imaging tests | CT scan, bone scan, MRI, PET scan, ultrasound, and X-ray examine bones and organs noninvasively |
Biopsy | Collecting a cell sample for laboratory testing — in most situations, the only way to definitively diagnose cancer |
Under the microscope, normal cells look uniform with similar sizes and orderly organization. Cancer cells look less orderly, with varying sizes and no apparent organization [2].
What are cancer stages?
Once cancer is diagnosed, doctors determine its extent, or stage, which guides treatment options and cure chances. Staging tests may include bone scans or X-rays to check whether the cancer has spread. Stages run from 0 through 4 (Roman numerals 0 through IV) — higher numbers indicate more advanced cancer, and some cancer types use letters or words instead [2].
What treatments are available for cancer?
Your treatment options depend on the type and stage of your cancer, your general health, and your preferences. You and your doctor weigh the benefits and risks of each option together [2].
What are the goals of treatment?
Cancer treatments have different objectives [2]:
Goal | What it means |
Cure | Treatment aims to cure the cancer so you can live a normal life span — possible or not depending on your situation |
Primary treatment | The first-line effort to remove the cancer completely or kill the cancer cells; surgery is the most common primary treatment for the most common cancers, though radiation or chemotherapy may lead for cancers sensitive to them |
Adjuvant treatment | Kills any remaining cells after primary treatment to reduce recurrence risk — commonly chemotherapy, radiation, or hormone therapy |
Palliative treatment | Relieves treatment side effects or cancer symptoms when a cure is not possible; can run alongside curative treatments |
What treatment tools do doctors have?
Treatment | How it works |
Surgery | Removes the cancer or as much of it as possible |
Chemotherapy | Uses drugs to kill cancer cells |
Radiation therapy | Uses high-powered energy beams such as X-rays and protons; delivered from outside the body (external beam) or inside (brachytherapy) |
Bone marrow transplant | Also called stem cell transplant; allows higher chemotherapy doses or replaces diseased marrow |
Immunotherapy | Also called biological therapy; helps the immune system recognize and attack cancer |
Hormone therapy | Removes or blocks hormones that fuel some cancers, such as breast and prostate cancer |
Targeted drug therapy | Focuses on specific abnormalities inside cancer cells that let them survive |
Clinical trials | Studies investigating new ways of treating cancer; thousands are underway [2] |
Can alternative medicine cure cancer?
No alternative cancer treatments have been proved to cure cancer. However, some may help you cope with side effects such as fatigue, nausea, and pain. Options found to be helpful include acupuncture, hypnosis, massage, meditation, relaxation techniques, and yoga. Always discuss these with your doctor to confirm they are safe and will not interfere with your treatment [2].
How do you cope with a cancer diagnosis?
A cancer diagnosis can change your life forever, and each person finds their own way of coping [2]. Practical ideas include learning enough about your cancer to make informed decisions about your care, keeping friends and family close for both practical and emotional support, and finding someone to talk with — whether a friend, family member, counselor, medical social worker, or cancer support group [2].
Preparing for a cancer appointment
If symptoms worry you, start with an appointment. A cancer diagnosis often leads to referrals to specialists such as oncologists, radiation oncologists, hematologists, and surgeons [2]. Come prepared:
Bring | Why it helps |
Pre-appointment restrictions | Some tests require diet or other restrictions in advance |
A written symptom list | Including anything that seems unrelated |
Key personal information | Major stresses or recent life changes |
Your family cancer history | Types of cancer, how each person is related to you, and age at diagnosis |
A full medication list | Including vitamins and supplements |
A companion | To remember details you might miss |
A question list | Prioritized from most to least important |
Questions you can ask your doctor
Questions worth asking include: What type and stage of cancer do I have? Will I need additional tests? What are my treatment options, and can they cure my cancer? What side effects can I expect? How soon must treatment begin? How will it affect daily life and work? Are there clinical trials available to me? What will determine my follow-up visits [2]?
Questions your doctor will ask you
Expect questions about when symptoms began, whether they are continuous or occasional, how severe they are, what improves or worsens them, any family cancer history, prior cancers and treatments, chemical exposures, tobacco use, and any hepatitis or HPV infection history [2].
The bottom line
Cancer is a family of diseases driven by gene mutations that remove the brakes on cell growth. It is common — about 2.1 million Americans will be diagnosed in 2026 — and it remains the second-leading cause of death worldwide, led by lung cancer. But the story is also one of progress: survival rates keep improving, most cases are diagnosed with a simple biopsy, and treatments range from surgery and chemotherapy to immunotherapy and targeted drugs.
The practical rules are simple. Know the warning signs and never ignore symptoms that persist. Know your risk — especially your family history. And act on what you can control: do not smoke, protect your skin, stay active, keep a healthy weight, drink in moderation, ask about hepatitis B and HPV vaccines, and keep up with cancer screening.
Know your body. Act early. Save this guide for the moment you need it most — and share it with someone you care about.
Frequently Asked Questions
What exactly is cancer?
Cancer is any one of a large group of diseases in which abnormal cells divide uncontrollably, infiltrate and destroy normal body tissue, and often spread throughout the body [1]. It is caused by mutations in the cell's DNA.
How does cancer start at the cellular level?
Mutations change a cell's genetic instructions so it grows rapidly, loses the tumor suppressor controls that tell normal cells when to stop, or fails to repair DNA errors [1]. Over time, enough mutations accumulate for the cell to become cancerous.
Is cancer genetic or inherited?
Most gene mutations happen after birth and are not inherited — caused by smoking, radiation, viruses, carcinogens, obesity, hormones, chronic inflammation, and lack of exercise. Only a small percentage of cancers come from mutations inherited from a parent [1] [9].
What are the early warning signs of cancer?
They include fatigue, a palpable lump, unintended weight changes, skin changes (including sores that will not heal and changing moles), bowel or bladder habit changes, persistent cough or hoarseness, persistent indigestion, unexplained pain, fevers or night sweats, and unexplained bleeding or bruising [1].
How is cancer diagnosed?
Through a combination of physical exam, laboratory tests (such as blood and urine tests), imaging (CT, MRI, PET, ultrasound, X-ray, bone scan), and — in most cases — a biopsy, which is the only definitive way to confirm cancer [2].
What do cancer stages mean?
Staging runs from 0 to 4 (Roman numerals 0–IV), with higher numbers meaning more advanced disease. The stage determines treatment options and the chances of a cure [2].
Can cancer be cured?
Many cancers can be cured, particularly when found early. Treatment goals include cure, primary removal or killing of the cancer, adjuvant therapy to prevent recurrence, and palliative care to relieve symptoms when a cure is not possible [2]. Survival rates for many cancers are improving [1].
What can I do to lower my cancer risk?
Do not smoke; limit sun exposure and use sunscreen; eat a diet rich in fruits, vegetables, whole grains, and lean proteins while limiting processed meats; exercise at least 30 minutes most days; maintain a healthy weight; moderate alcohol; get recommended cancer screenings; and ask about hepatitis B and HPV vaccinations [1].
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