What Causes Cancer?
Cancer develops when changes accumulate in genes that control cell growth and division. Some arise during cell division, some are linked to exposures or infections, and some can be inherited.
Short answer
Cancer begins when genetic changes allow cells to grow or behave abnormally. Some changes are linked to tobacco smoke, UV radiation or certain infections; other changes can be inherited. There is usually no single cause that can be assigned to one person’s cancer.
What we know
Older age, tobacco, alcohol, ultraviolet and ionizing radiation, certain infections, excess body weight and some environmental or occupational exposures are associated with higher risks of particular cancers.
What cannot be determined
A risk factor alone cannot explain why a specific person developed cancer, and many exposed people never develop it. Cancer can also occur without an obvious known risk factor.
What do the studies say together?
Taken together, the studies point to a consistent picture: cancer does not have one universal cause. Population analyses show that tobacco, alcohol, certain infections, excess body weight, ultraviolet radiation and other established exposures account for a substantial share of cancer burden, but the size of each contribution varies by cancer type and population. These estimates describe groups, not individual patients, so they usually cannot identify the single cause of one person’s cancer. Inherited changes and changes that arise as cells divide also contribute to cancer development.
What does this mean for you?
The more useful question is often not which single thing caused a cancer, but which known risks can be reduced and which prevention or screening steps are appropriate based on age, family history, health context and established medical guidance.
What did the studies find?
Original sources: Each study below links to its original record with a descriptive title.
Study 1: Global and regional cancer burden attributable to modifiable risk factors — 2022 data
- What did it study?
- Researchers used GLOBOCAN data for 36 cancer sites in 185 countries and estimated burden associated with 30 modifiable risk factors including smoking, alcohol, excess body weight, infections, UV exposure, air pollution and occupational exposures.
- What did it find?
- The study estimated that about 7.1 million of 18.7 million new cancer cases worldwide in 2022 were attributable to the 30 measured factors; smoking, infections and alcohol were leading contributors.
- What are its limits?
- The fraction depends on the selected risks, historical exposure estimates and large differences between countries, sexes and cancer types. It is not a personal risk estimate.
- What can we take from it?
- Very broad recent evidence supporting the importance of modifiable risks for population prevention while retaining the limits of modelling.
Study 2: Global cancer burden attributable to risk factors — GBD 2019
- What did it study?
- A global systematic analysis estimated cancer deaths and disability attributable to behavioural, environmental, occupational and metabolic risks across 82 risk–cancer outcome pairs.
- What did it find?
- The analysis estimated about 4.45 million cancer deaths in 2019 were attributable to the risk factors it measured; smoking was the largest contributor.
- What are its limits?
- These are population-level attributable estimates based partly on modelling. They cannot identify the cause of an individual person’s cancer.
- What can we take from it?
- Strong population evidence that established risk factors contribute substantially to cancer burden, without supporting individual causal attribution.
Study 3: Cancers attributable to modifiable risk factors — a roadmap for prevention
- What did it study?
- The review examined evidence on tobacco, occupational exposures, infections, air pollution, alcohol, diet and obesity and how epidemiology estimates the preventable share of cancer.
- What did it find?
- It concludes that known modifiable factors account for an important part of cancer burden and prevention can avert cases, while knowledge of avoidable causes remains incomplete.
- What are its limits?
- This is an interpretive review rather than a new meta-analysis producing one risk estimate; its value is the overall evidence map, not individual prediction.
- What can we take from it?
- Strong support for the overall picture: cancer has multiple causes, prevention is partly possible, and science cannot assign one cause to every individual case.
Study 4: Global cancer burden attributable to infections in 2018
- What did it study?
- Using GLOBOCAN 2018 data, researchers estimated cancer cases attributable to ten infectious agents classified as human carcinogens, by country, sex and age.
- What did it find?
- About 2.2 million cancer cases in 2018 were estimated to be attributable to infections; H. pylori, HPV, hepatitis B and hepatitis C were major contributors.
- What are its limits?
- The analysis concerns infection-related cancers only and does not explain the causes of other cancers; burden also varies greatly by region.
- What can we take from it?
- Strong evidence that specific infections are preventable causes of an important share of cancer worldwide.
Study 5: Body mass index and cancer risk — umbrella review of meta-analyses
- What did it study?
- The umbrella review brought together 18 systematic reviews and meta-analyses on body mass index and multiple cancer types, including dose–response evidence.
- What did it find?
- Obesity was associated with higher incidence of several cancers, including kidney, endometrial, oesophageal, colorectal, liver and gallbladder cancers, with dose–response patterns for some sites.
- What are its limits?
- Most underlying evidence was observational and can be affected by confounding and differences in exposure definitions and follow-up. Associations are not equally strong for every cancer.
- What can we take from it?
- Good synthesis evidence that excess body weight is an established risk factor for several, but not all, cancers.
General institutional references
Persistent symptoms or concerns about personal risk should be discussed with a qualified health professional. This page cannot diagnose an individual or determine a personal risk level.
Editorial review
The Reffaq Cancer Magazine team prepared this page using official health sources and original studies appropriate to the question. The source-review date appears on the page, and when a qualified medical professional reviews the content we identify the reviewer and their role clearly.