Cancer is a disease, but more precisely, it is a group of more than 100 related diseases that all share one core feature: cells in the body grow uncontrollably and can spread to other tissues. The reason it’s classified as a group rather than a single disease is that cancer can start in virtually any organ or tissue, behave differently depending on where it forms, and require entirely different treatments from one type to the next.
Why Cancer Is Called a Group of Diseases
When people hear “cancer,” they often think of it as one thing. In reality, lung cancer and leukemia have about as much in common as pneumonia and a urinary tract infection. Both involve infections, but the causes, symptoms, and treatments are completely different. Cancer works the same way. Each type is named for the organ or tissue where it starts, and each has its own biology, risk factors, and outlook.
There are six major categories based on where the cancer originates. Carcinomas form in the tissues that line organs and surfaces, both inside and outside the body, and account for the majority of cancers. Sarcomas arise in connective and supportive tissues like bone, muscle, cartilage, and fat. Leukemias are cancers of the bone marrow that affect blood cell production, often causing an overproduction of immature white blood cells. Lymphomas develop in the lymphatic system, the network of nodes and vessels involved in immune defense. Myelomas start in the plasma cells of bone marrow. A sixth category covers mixed types that span more than one tissue.
Even within a single category, the variation is enormous. Breast cancer alone includes several distinct subtypes that respond to different therapies. Each person’s cancer carries a unique combination of genetic changes, and even cells within the same tumor can differ from one another genetically.
What Goes Wrong Inside the Cell
Your body replaces billions of cells every day through a tightly regulated cycle of growth, division, and death. Cancer begins when that cycle breaks down. Normally, built-in checkpoints catch mistakes in DNA and either repair them or trigger the damaged cell to self-destruct, a process called apoptosis. When the genes that control those checkpoints stop working correctly, damaged cells survive and keep dividing.
Two types of genes matter most. Growth-promoting genes can get stuck in the “on” position, pushing cells to multiply without the usual signals telling them to stop. Meanwhile, tumor suppressor genes, especially one called p53, act as the brakes. When p53 is functioning, it can pause cell division, initiate DNA repair, or force a badly damaged cell to die. Mutations that knock out p53 remove those brakes entirely.
Cancer cells also cheat death in another way. Normal cells can only divide a limited number of times before their protective chromosome caps (telomeres) wear down and the cell dies. Cancer cells activate an enzyme that rebuilds those caps after every division, effectively making the cell immortal. This is why tumors can grow indefinitely if left unchecked.
Beyond just growing, cancer cells develop additional capabilities over time: they recruit new blood vessels to feed the tumor, reprogram their metabolism to fuel rapid growth, dodge immune cells that would normally destroy them, and eventually invade surrounding tissues or travel through the bloodstream to distant organs. That last step, metastasis, is what makes cancer particularly dangerous.
What Causes It
Only 5 to 10 percent of all cancer cases trace back to inherited genetic defects. The remaining 90 to 95 percent are rooted in environmental and lifestyle factors. That doesn’t mean those cancers are anyone’s “fault,” but it does mean the majority of cancer risk comes from exposures accumulated over a lifetime: tobacco use, diet, obesity, chronic infections, radiation, and chemical exposures among them.
The distinction matters because it means cancer is not purely a genetic lottery. Many of the DNA mutations that drive cancer are acquired over decades rather than inherited at birth. This is one reason cancer becomes more common with age. The longer cells divide, the more opportunities there are for errors to accumulate and for repair mechanisms to fail.
How Cancer Is Measured and Staged
When cancer is diagnosed, doctors assess how far it has progressed using a system built around three factors. The first is the size and extent of the primary tumor. The second is whether cancer has reached nearby lymph nodes, and if so, how many. The third is whether cancer has spread to distant parts of the body. Each factor is scored on a scale, and the combination determines an overall stage, typically ranging from stage I (small, localized) to stage IV (spread to distant organs).
Staging directly shapes treatment decisions and gives a general sense of prognosis, though outcomes vary widely even within the same stage depending on the specific cancer type and its genetic profile.
A Shift Toward Genetic Classification
Traditionally, cancer has been classified by where it starts: breast cancer, colon cancer, lung cancer. That’s changing. Advanced genetic sequencing now allows doctors to identify the specific mutations driving a tumor, regardless of its location. The FDA has approved targeted therapies that treat cancers based on their molecular profile rather than their organ of origin. A drug designed to target a particular gene mutation might work for a patient with lung cancer and another with thyroid cancer, as long as both tumors share that mutation.
This approach, sometimes called tissue-agnostic treatment, reflects a growing understanding that two cancers in the same organ can be fundamentally different diseases, while two cancers in completely different organs can share the same molecular machinery. As this field advances, where a cancer started will matter less than what is driving it at the genetic level.
The Global Scale of Cancer
In 2022, there were an estimated 20 million new cancer cases and 9.7 million cancer deaths worldwide. Lung cancer was the most common diagnosis, with 2.5 million new cases, followed by female breast cancer (2.3 million), colorectal cancer (1.9 million), prostate cancer (1.5 million), and stomach cancer (970,000).
The deadliest cancers don’t perfectly mirror the most common ones. Lung cancer led in deaths as well, at 1.8 million, but liver cancer ranked third in mortality despite not making the top five in new diagnoses, reflecting how difficult it is to detect and treat at later stages. Breast cancer, while second in incidence, ranked fourth in deaths, partly because screening catches many cases early when treatment is most effective.
These numbers underscore why cancer is considered one of the leading global health challenges. It is not a single epidemic but a collection of related diseases, each with its own patterns of risk, detection, and survival.