Interactive guide
cancer /ˈkansə/ · noun
A disease in which some of the body's cells grow uncontrollably and spread to other parts of the body.
From the Greek karkinos, "crab". Hippocrates used the word for tumours, and the Roman physician Celsus turned it into the Latin cancer.
Just scroll. Every picture moves with you, and you collect key terms as you go.
Level 1 · The basics
Your body is built from trillions of cells. Each one is a tiny living unit with a control centre, the nucleus, which holds the cell's instructions: its DNA.
Normally, cells grow and divide to make new cells only when the body needs them, for example to replace worn-out cells or repair damage.
When cells grow old or become damaged, they die and new cells take their place. This built-in self-destruct is called apoptosis, or programmed cell death.
Level 1 · The basics
Cancer starts when this orderly process breaks down. A cell picks up changes in its DNA, called mutations. Most damage gets repaired, but sometimes changes build up.
Cancer Research UK says it takes about six before a normal cell turns into a cancer cell. Watch the counter.
That cell now keeps dividing when it shouldn't, and doesn't die when it should. Its copies pile up into a lump called a tumour.
Each round of division doubles the count: 1, 2, 4, 8. After just ten rounds there are more than a thousand cells.
Level 1 · The basics
Cancer cells behave differently from normal cells in ways that let them grow out of control and invade. The US National Cancer Institute lists eight differences. Scroll, and watch each rule break.
NormalGrows only when it receives a signal to grow.
CancerGrows with no signal at all.
NormalStops dividing, or dies by apoptosis, when told to.
CancerIgnores the signals to stop or to die.
NormalStops growing when it meets other cells, and mostly stays put.
CancerInvades nearby areas and spreads to other parts of the body.
NormalUses the blood vessels already around it.
CancerTells blood vessels to grow towards the tumour, bringing oxygen and nutrients and removing waste.
NormalDamaged or abnormal cells are normally removed by the immune system.
CancerHides from it.
NormalImmune cells attack what doesn't belong.
CancerCan convince immune cells to protect the tumour instead of attacking it.
NormalKeeps its chromosomes intact.
CancerGains duplications and deletions of chromosome parts. Some have double the normal number.
NormalMakes energy from nutrients in the usual way.
CancerRelies on different nutrients and makes energy differently, which helps it grow faster.
Every broken rule is also a weakness. Many modern treatments aim at one of them, as you'll see in Levels 4 and 5.
Level 1 · The basics
A tumour is a lump of tissue. Many cancers form solid tumours, but cancers of the blood, such as leukaemias, generally do not. And not every tumour is cancer.
A benign tumour does not spread into nearby tissue and usually doesn't grow back once removed. It is not cancer, although some, like certain brain tumours, can still be serious because of their size or where they are.
A malignant tumour is cancer. It grows into nearby tissue, and cells can break away and travel through the blood or lymph to form new tumours elsewhere.
Level 1 · The basics
Under a microscope, doctors can sometimes see tissue changing before cancer forms. This is a lining tissue: one neat layer of matching cells sitting on a thin sheet called the basement membrane.
Cells multiply faster than usual, but each one still looks normal. This is not cancer.
The extra cells look abnormal and lose their orderly arrangement. The more abnormal they look, the greater the chance of cancer.
Abnormal cells fill the layer but have not broken through the membrane. It is often called stage 0. It isn't cancer yet, but it is usually treated because it may become cancer.
Abnormal cells break through the basement membrane into the tissue below, where blood and lymph vessels run. The cancer is now invasive.
Level 2 of 7 unlocked
Zoom into the code: the genes involved, and where the changes come from.
Level 2 · How it starts
Cancer is a genetic disease. It is caused by changes to genes that control how cells grow and divide. Let's zoom in, from a whole organ down to a single letter of DNA.
We start with the lungs. Lung cancer is the most common cancer in the world.
Zoom into the lining of an airway and you see cells packed side by side. Carcinomas, the most common cancers, start in linings like this.
A single cell, about a hundredth of a millimetre across. Its control centre is the nucleus.
The nucleus holds most of the cell's DNA, wound up tightly. When a cell gets ready to divide, the DNA packs into compact shapes.
DNA is packed into 46 chromosomes, in 23 pairs, one set from each parent. Genes sit along them, like the pink band.
DNA is a code written in four letters, A, T, C and G, about 3 billion letters in each set. A gene is a stretch of that code, often the recipe for a protein. People have about 19,900 protein-coding genes.
Change one letter and the recipe can change. That is a mutation. If it lands in a gene that controls growth, the cell may stop following the rules.
Level 2 · How it starts
The changes behind cancer usually affect three kinds of genes. Proto-oncogenes are the accelerator: they drive growth. A change can jam them on, turning them into cancer-causing oncogenes.
Tumour suppressor genes are the brakes that slow or stop cell division. When they are switched off, nothing tells the cell to stop.
DNA repair genes fix damage. When they fail, new mistakes and chromosome changes pile up faster.
One fault is rarely enough. It usually takes several together before a cell becomes cancerous. Watch how each one makes the cell count climb faster.
Level 2 · How it starts
Gene changes come from three places. The first is copying errors. Every time a cell divides it copies its DNA, and now and then it makes a mistake.
Some changes are caused by damage from things around us, such as the chemicals in tobacco smoke and the sun's ultraviolet rays. Person B collects more of them.
Some people are born with a change passed down from a parent, so they start life one step along. Up to 10% of all cancers may be caused by inherited genetic changes.
Most changes collect slowly over a lifetime, and the body gets less good at removing damaged cells as we age. That is part of why cancer is more common in older people. Even then, a tumour can take many years to grow big enough to notice.
Level 3 of 7 unlocked
How cancer travels through the body, and how more than 100 types are named.
Level 3 · Spread and types
Spread to another part of the body is called metastasis. Most people who die of cancer die of metastatic disease. It happens in steps. First, cancer cells invade nearby normal tissue.
They move through the walls of nearby lymph nodes or blood vessels.
They are carried through the bloodstream or the lymphatic system to other parts of the body.
They get stuck in a small blood vessel somewhere far away.
They push through the vessel wall into the tissue around it, and grow until a tiny tumour forms.
The new tumour makes blood vessels grow towards it. It keeps its first name: breast cancer that spreads to the lung is metastatic breast cancer, not lung cancer. The most common places for spread are bone, liver and lung.
Level 3 · Spread and types
There are more than 100 types of cancer. Most are named after the organ or tissue where they start, such as lung or breast cancer. Doctors also group them by the kind of cell they start in. Here are the main families.
The most common family. Carcinomas start in epithelial cells, which line the inside and outside surfaces of the body. Types include adenocarcinoma, basal cell, squamous cell and transitional cell carcinoma.
Sarcomas start in bone and soft tissues such as muscle, fat, blood vessels, lymph vessels and tendons. Osteosarcoma is the most common cancer of bone.
Starts in the blood-forming tissue of the bone marrow. Leukaemias generally don't form solid tumours. Instead, abnormal white blood cells build up and crowd out normal blood cells.
Lymphomas start in lymphocytes, immune cells found in lymph nodes, lymph vessels and other organs. The two main types are Hodgkin and non-Hodgkin lymphoma.
Myeloma starts in plasma cells, immune cells that make antibodies. The abnormal cells build up in the bone marrow and form tumours in bones across the body.
Melanoma starts in melanocytes, the cells that make the pigment that gives skin its colour. Most form on the skin, but they can also form in other pigmented tissue, such as the eye.
These tumours are named after the type of cell they start in and where they first formed. They can be benign or malignant.
Germ cell tumours start in the cells that make sperm or eggs. Neuroendocrine tumours start in cells that release hormones. Carcinoid tumours are a type of neuroendocrine tumour, usually slow-growing and mostly found in the gut.
Level 4 of 7 unlocked · advanced
The ideas researchers use to understand every cancer: hallmarks, the guardian gene, the immune system and evolution.
Level 4 · The deeper science
In 2000, the scientists Douglas Hanahan and Robert Weinberg proposed that cancers share a set of abilities that cells gain on the way to becoming cancer. They updated the list in 2011, and Hanahan suggested more in 2022.
The best-known version has eight hallmarks and two enabling traits.
Sustaining growth signals: the cell keeps sending itself "go" signals. Evading growth suppressors: it ignores the brakes.
Resisting cell death: apoptosis is switched off. Replicative immortality: normal cells can only divide a limited number of times, but cancer cells can keep going.
Inducing angiogenesis: growing new blood vessels to feed the tumour. Invasion and metastasis: breaking out and spreading.
Reprogramming energy metabolism: using fuel differently to grow fast. Avoiding immune destruction: escaping the immune cells that should remove it.
Two traits help cells gain the rest. Genome instability and mutation makes changes happen faster. Tumour-promoting inflammation creates surroundings that help the tumour grow.
Hanahan proposed four more candidates: cells changing identity (phenotypic plasticity), epigenetic reprogramming without mutation, the microbes living in and on us (polymorphic microbiomes), and senescent cells.
Treatments can aim at single hallmarks. Targeted therapies can block the signals that tell cancer cells to grow. Angiogenesis inhibitors cut off a tumour's new blood supply. Immunotherapy helps the immune system see the cancer again.
Level 4 · The deeper science
One brake matters more than most. The TP53 gene makes a protein called p53, a tumour suppressor. It sits in the nucleus, binds directly to DNA and watches for damage.
When DNA is damaged, p53 decides what happens next. If the damage can be fixed, it switches on genes that repair it.
If the damage is too great, p53 stops the cell from dividing and signals it to self-destruct by apoptosis. That's why p53 is called the guardian of the genome.
If TP53 itself is changed, p53 can no longer stop damaged cells. They keep dividing, their DNA damage builds up, and tumours can grow.
Changes in TP53 are the most common genetic changes in human cancer, found in about half of all cancers. These changes happen during a person's life and are found only in the cancer cells.
Level 4 · The deeper science
A tumour is more than cancer cells. They are surrounded by immune cells, connective-tissue cells called fibroblasts, signalling molecules and blood vessels. This is the tumour microenvironment, and cancer cells can change it to help them grow and spread.
The immune system normally finds and removes damaged or abnormal cells. T cells are among its hunters.
To stop the immune system harming healthy tissue, T cells carry checkpoint proteins such as PD-1. When PD-1 binds to its partner, PD-L1, the T cell gets an "off" signal. Some tumours make lots of PD-L1 to switch T cells off.
Immune checkpoint inhibitors are drugs that block this binding. With no "off" signal, the T cell can kill the cancer cell.
Checkpoint inhibitors are one kind of immunotherapy, treatment that helps the immune system fight cancer. They work well for some people and some cancers, and researchers are studying why they don't work for everyone.
Level 4 · The deeper science
Every person with cancer has a unique combination of DNA changes in their cancer. Two people with the same type of cancer can have different sets of changes, like two different barcodes.
Even two cancer cells in the same tumour can carry different DNA changes. A tumour is really a mix of groups of cells. Here: groups A, B and C.
A targeted drug destroys the cells that carry its target. Groups A and B carry it. Group C does not.
The survivors multiply and the tumour returns. This is one way a treatment can work and then stop working. Resistance can also happen when the target itself changes.
Not every change is to the code itself. Epigenetic changes alter how DNA is packed, which changes how much protein a gene makes, and they may be reversible. Doctors use biomarker testing to see what a tumour carries, and combine treatments.
Level 5 of 7 unlocked
How cancer is found, measured and treated.
Level 5 · Finding and treating
Many cancers can be cured if they are found early and treated well. Early diagnosis has three parts: knowing the symptoms, getting checked, and getting to treatment quickly.
Screening looks for cancer, or changes before cancer, in people without symptoms. Examples include the HPV test for cervical cancer and mammograms for breast cancer in women aged 50 to 69. It works for some cancers, not all.
Scans show where a cancer is. In a PET scan, a small amount of radioactive glucose is injected into a vein and travels around the body.
Cancer cells are often more active and take up more glucose, so they show up brighter. Some healthy organs, like the brain and bladder, always light up, which is why specialists read the scans.
To confirm cancer, a small sample of tissue, a biopsy, is examined under a microscope by a pathologist.
Level 5 · Finding and treating
Staging describes how much cancer there is and where. The TNM system starts with T, the main tumour, scored T1 to T4. A higher number means a larger tumour or more growth into nearby tissue.
N describes nearby lymph nodes. N0 means no cancer in them. N1 to N3 means more nodes contain cancer.
M says whether cancer has spread to distant parts of the body: M0 no, M1 yes. Together they give a stage. Stage 0 means abnormal cells that haven't spread. Stages I to III mean a larger tumour or more spread nearby. Stage IV means distant spread.
Grade describes how abnormal the cells look under a microscope, from grade 1, close to normal, to grade 4, the most abnormal. High-grade cancers tend to grow and spread faster.
Level 5 · Finding and treating
Treatment depends on the type of cancer, where it is and how far it has spread. Treatments are often combined. Here are the main ones.
A surgeon removes the cancer from the body. Surgery works best for solid tumours that are contained in one area.
Radiotherapy uses high doses of radiation to kill cancer cells and shrink tumours by damaging their DNA. Beams can be aimed from several angles so they meet at the tumour.
Drugs that destroy or stop the growth of cancer cells and other fast-dividing cells. That is why some healthy fast-growing cells, such as those in hair, can be affected too.
Targeted therapy drugs aim at the molecular changes that help cancer cells grow, divide and spread. Hormone therapy slows or stops cancers that use hormones to grow, such as some breast and prostate cancers.
Helps the immune system fight cancer, for example by releasing the PD-1 brake you met in chapter 13.
Others include blood stem cell transplants, hyperthermia (heat) and photodynamic therapy (a drug switched on by light). Palliative care relieves pain and other problems at any stage. The WHO says relief is possible for more than 90% of people with advanced cancer.
Level 6 of 7 unlocked
The size of cancer worldwide, from the World Health Organization and its cancer agency, IARC.
Level 6 · The numbers
About 20 million people worldwide were diagnosed with cancer in 2022, according to the International Agency for Research on Cancer (IARC).
About one in five people develop cancer during their lifetime.
Cancer caused nearly 10 million deaths in 2024. That is close to one in every six deaths worldwide.
About 53.5 million people were alive within five years of a cancer diagnosis in 2022. Cancer also affects the young: about 400,000 children and teenagers develop it each year.
More than 35 million new cases are predicted in 2050, 77% more than in 2022. The main reasons are populations growing and ageing, and changes in people's exposure to risk factors.
Level 6 · The numbers
Lung cancer is the most common cancer in the world, with 2.6 million new cases in 2024, followed by breast, colon and rectum, and prostate.
The deaths list looks different. Lung is still first. Liver cancer, not in the top six for new cases, is third for deaths. Non-melanoma skin cancer is common but isn't on the deaths list at all.
How common a cancer is and how deadly it is are different questions. Finding cancer early and treating it well changes the second.
Level 7 of 7 unlocked · final
What lowers the risk, and when to see a doctor.
Level 7 · Prevention and signs
The World Health Organization estimates that about 38% of cancers can currently be prevented, by avoiding risk factors and using proven prevention.
Nearly a quarter of cancer deaths are due to tobacco use, alcohol, high body mass index, eating few fruits and vegetables, and lack of physical activity. Air pollution is also an important risk factor for lung cancer.
About 10% of cancers diagnosed in 2022 were caused by infections such as HPV, hepatitis B and C, H. pylori and Epstein–Barr virus. Vaccines against HPV and hepatitis B help prevent some of them.
The prevention steps in the WHO cancer fact sheet. Each one ticks as you pass it.
Level 7 · Prevention and signs
Cancer can cause many different symptoms. Most of the time they turn out to be something less serious, but it is still worth getting checked. These are some of the signs Cancer Research UK lists.
See a doctor about a cough that lasts more than three weeks, or a mouth ulcer or patch that hasn't healed after three weeks. Tell them about anything that isn't normal for you. You don't need to know whether it's serious first, and you aren't wasting their time. If it carries on, go back.
Summary
Guide complete
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Sources
Only official health bodies and the original research papers were used. Facts were checked on 24 October 2026.
Outline · which source backs which part
For education only. This guide cannot diagnose anything or replace advice from your own doctor or care team. Illustrations simplify real biology.