Cancer
Cancer is a large group of diseases in which some cells grow out of control and can invade nearby tissue or spread to other parts of the body. It can begin in many tissues, has many causes and risk factors, and is treated with different combinations of surgery, medicines, radiation, immunotherapy, and supportive care.
What cancer is
Cancer is not one disease. It is a broad name for diseases in which abnormal cells grow when they should not, avoid normal control signals, and may invade nearby tissue. Some cancers form solid tumors. Others, such as many leukemias, involve blood-forming tissues and may not form a single lump.
How cancer starts
Cancer usually begins when DNA changes affect genes that control cell growth, repair, death, and division. Some changes are inherited, but many are acquired during life. They can arise from aging, random copying errors, tobacco smoke, ultraviolet radiation, infections, chemicals, inflammation, radiation exposure, or other factors. One DNA change is usually not enough; cancer often develops through multiple steps.
Tumors and metastasis
A tumor is an abnormal mass of tissue. Benign tumors can grow but do not invade nearby tissues or spread to distant sites in the way malignant tumors can. Cancerous tumors can invade surrounding tissue, enter blood or lymph vessels, and establish new tumors elsewhere. That distant spread is called metastasis and is a major reason cancer can become life-threatening.
Types of cancer
Cancers are often named for where they start or the cell type involved. Carcinomas begin in epithelial tissues such as skin, lung, breast, colon, or prostate lining cells. Sarcomas begin in bone, muscle, fat, or connective tissue. Leukemias affect blood-forming tissues. Lymphomas and myeloma involve immune-system cells. Brain and spinal cord tumors have their own classification patterns.
Risk factors and prevention
Cancer risk is shaped by age, inherited genetics, infections, environment, occupational exposures, behavior, and social conditions. Not every cancer can be prevented, and risk is not the same as blame. Still, public-health evidence supports risk reduction through avoiding tobacco, limiting alcohol, protecting skin from excess ultraviolet exposure, vaccination against HPV and hepatitis B when recommended, physical activity, healthy weight, safer workplaces, and reduced exposure to some pollutants.
Screening and early detection
Screening means looking for cancer or precancerous changes before symptoms appear. Screening can reduce harm for some cancers when the test, age group, and follow-up plan are well matched. It can also cause false alarms, missed cancers, overdiagnosis, or unnecessary procedures. That is why screening recommendations differ by cancer type, age, risk, and country.
Diagnosis and staging
Cancer diagnosis may involve physical exams, imaging, blood tests, endoscopy, biopsy, pathology, genetic or molecular testing, and other studies. Staging describes how large the cancer is and whether it has spread. Grading describes how abnormal cells look or behave. These details help clinicians estimate outlook and choose treatment.
Treatment approaches
Cancer treatment may include surgery, radiation therapy, chemotherapy, targeted therapy, immunotherapy, hormone therapy, stem cell transplant, active surveillance, palliative care, or combinations of these. The best approach depends on cancer type, stage, molecular features, patient health, goals of care, and treatment access. Two people with the same general cancer name may need very different plans.
Living with and after cancer
Cancer care includes more than attacking tumor cells. People may need help with pain, fatigue, nausea, fertility, nutrition, infection risk, anxiety, work, family life, finances, rehabilitation, and long-term follow-up. Survivorship can include monitoring for recurrence, managing late effects of treatment, and rebuilding everyday routines after diagnosis.
Why it matters
Cancer matters because it connects cell biology, public health, medicine, families, and society. It is a leading cause of illness and death worldwide, yet outcomes vary widely by cancer type, stage, prevention, screening access, treatment advances, and health-system resources. Understanding cancer helps readers separate broad principles from individual medical decisions.