This section assumes knowledge from the sections: DNA, Cancer, Human cells, Basic biology, Mitosis and meiosis.
The principal cause of cancer is mutation, which can be caused by a number of things. It is impossible to live a life with 0% chance of developing cancer. Practically anything that interacts with DNA can cause cancer. Even though the human body is very well equipped to repair damage, issues usually arise when DNA is damaged repeatedly or when the body’s repair systems are not functioning properly. When cells repair damage it is likely a small portion of these mutations will be repaired incorrectly, perhaps the damaged DNA strand was mistaken as healthy and the other strand was changed to match it, perhaps the damage broke both sides of the strand leaving the cell without indication of the previous configuration. If a cell repairs this damage in a way that leaves it different to the original but still structurally sound this damage becomes a permanent mutation.
There is evidence to suggest that cells experience around 100,000 DNA lesions each day(1). This research highlights how adept cells are at repairing damage, but also how many opportunities there are for the repair process to go wrong. Cancer usually develops after many mutations build up until they eventually effect a gene that helps regulate cell growth. Some examples of causes of cancer include endogenous factors, inherited genetics, carcinogenic chemicals, viruses and radiation:
Endogenous factors. Not all cancers are impacted by any outside source. Epidemiology has shown that only about 70-80% of cancer cases can be linked to an exogenous component (2). DNA damage can occur due to endogenous factors such as reactive metabolites (by-products created when the body breaks down various molecules) and hydrolysis. This damage is largely unavoidable and is simply a part of the risk of living. These mutations can make genes more susceptible to other causes of cancer or be the entire cause of carcinogenesis (1).
Inherited genetics. Cancer itself cannot be passed down to children, this is because if gametes are mutated in a way that causes them to replicate in an improper way it would not be able to develop into a viable embryo. However it is possible to pass on genetics that increase the overall risk of cancer. These genes usually do not simply cause cancer, but rather inhibit or weaken the cellular processes to do with repairing/protecting DNA. A child only inherits half the genetic material from each parent, and only about 10% of cancers can be linked to an inherited gene. If concerned, talk to your doctor as there are genetic tests for these conditions and prophylactic treatments.
Carcinogenic chemicals. Carcinogenic is an adjective used to describe anything that has the potential to cause cancer. Everything in this list could be referred to as carcinogenic, (carcinogenic genes, carcinogenic endogenous processes, etc) however this section is specifically focusing on carcinogenic chemicals. There are a variety of substances that can interact with, and damage DNA. There are a number of commonly known sources of carcinogenic chemicals such as cigarettes and asbestos and a huge amount of other lesser known examples. It is often hard to determine and prove something causes cancer as there are so many variables when studying the cause of cancer.
For example, smoking was popular throughout America for most of the twentieth century. A multitude of studies were conducted to investigate negative health effects of cigarettes (such as lung cancer), however the cigarette industry was able to cast doubt on the research due to the findings being chance or correlation based. A direct and immediate causal link was only discovered late in the 20th century and without one, proving that high quantities of tobacco smoke, over a prolonged period of time, increased the risk of lung cancer, was an annoyingly shaky position to convince the public of (3). For the record, this paper does not recommend tobacco consumption due to its addictive properties and the multitude of health conditions that exposure can cause.
Other carcinogenics are currently under research, such as red meat. According to the World Health Organisation (WHO) red meat is currently classified as a group 2A carcinogen, this means that there is limited research and other causes/biases/variables could not be ruled out. With that said the WHO recommends limiting red meat consumption while also admitting that red meat has a variety of proven health benefits and no clear safe consumption level has been determined. All of this to say many things can have impacts on cancer and discerning when to actually adjust lifestyle choices can be hard to do effectively. (4)
Radiation. Radiation is defined as energy moving in space or through a medium using particles or waves. This energy can damage DNA by altering the chemical structure. Subtracting or adding energy to the complex web of bonds and forces in DNA can disrupt the entire structure, causing mutations.
The most common type of radiation is electromagnetic radiation. Electromagnetic radiation is one of the only types of radiation people are likely to encounter everyday, meaning that it is also the only type of radiation people are likely to encounter without prior training. This lack of training can lead to a number of myths about the dangers of household appliances such as phones, microwaves etc. Electromagnetic radiation is defined as waves of energy, depending on the frequency of the waves they are classified and used differently. Fig 1 shows different types of electromagnetic radiation. While things with lower energy such as visible light, infrared, microwaves and radio do not have enough energy to alter the structure of molecules (non ionizing), higher energy waves are able to (ionizing). Examples of ionizing radiation include ultraviolet, X rays and gamma radiation are known causes of cancer as they have enough energy to alter DNA.
It is important to remember that in medical settings X rays and gamma rays are used for treatments and diagnosis. The risk of developing cancer from measured amounts of radiation within clinical settings are usually far outweighed by the benefits of such a procedure. Talk to your doctor if you are concerned about the risk of a procedure. (5)
Note: more on the use of radiation in diagnosis here. And more on the use of radiation in treatment here.
Viruses. Viruses are a type of infectious pathogen, they differ from other forms of pathogens because viruses are not actually alive. Viruses are unable to perform the functions necessary to be considered alive, some of these functions include metabolism, homeostasis, response to stimuli etc. Viruses function by entering host cells and introducing its own genetic material into those cells. This genetic material codes for the replication of itself. Because cells are usually unable to tell the difference they will produce the proteins as the code instructs. This means viruses effectively hijack a cell’s protein synthesis pathway to produce more of itself.
Some viruses can have carcinogenic effects, these effects can be direct or indirect causes of cancer. For example some viruses insert DNA into the actual genome of the cell, this can change the original code and disrupt cell function. Some of these viruses have onco-genes meaning that they directly cause carcinogenesis. Other viruses can be indirect causes as they may only insert RNA into the cytoplasm, this gets read into proteins but does not disrupt the actual genome of the cell. However cells infected by viruses can function poorly and die or be killed by the immune system that recognises the infection. This means new cells must replicate to take their place, this leads to rapid replication and sometimes improper replication causing scar tissue, this scarring can result in carcinogenesis.
Note: there are other pathogens that have been linked to cancer, however most other carcinogenic living pathogens cause cancer by producing and releasing carcinogenic toxins. This is already covered above in the carcinogenic section.
| 1. | De Bont R, van Larebeke N. Endogenous DNA damage in humans: a review of quantitative data. Mutagenesis. 2004;19(3):169-185. doi:10.1093/mutage/geh025 |
| 2. | Doll R, Peto R. The causes of cancer: quantitative estimates of avoidable risks of cancer in the United States today. Journal of the National Cancer Institute. 1981;66(6):1191-1308. Accessed July 11, 2026. https://pubmed.ncbi.nlm.nih.gov/7017215/ |
| 3. | National Center for Chronic Disease Prevention and Health Promotion (US) Office on Smoking and Health. Fifty Years of Change 1964–2014. Nih.gov. Published 2014. Accessed July 11, 2026. https://www.ncbi.nlm.nih.gov/books/NBK294310/ |
| 4. | World Health Organization. Cancer: Carcinogenicity of the Consumption of Red Meat and Processed Meat. World Health Organization. Published October 26, 2015. Accessed July 11, 2026. https://www.who.int/news-room/questions-and-answers/item/cancer-carcinogenicity-of-the-consumption-of-red-meat-and-processed-meat |
| 5. | Cancer Council. Can radiation from medical tests cause cancer? www.cancer.org.au. Accessed July 11, 2026. https://www.cancer.org.au/iheard/can-radiation-from-medical-tests-cause-cancer |