Dr. Deenadayalan T explains how cancer develops (Tamil).
Cancer is caused by damage to the genes that control how cells grow and die. Normally, cells divide when needed and self-destruct when old or damaged. When the genes governing that process are damaged, cells keep multiplying and stop dying — and that uncontrolled growth is cancer. The damage accumulates over years, driven by four broad forces: lifestyle, environment, infection and heredity.
Cancer does not appear overnight. In most cases it is the end point of a process that has been running quietly for a decade or more.
How a normal cell works
The human body contains roughly 30 to 37 trillion cells. Every day, billions of them divide to replace cells that are worn out or damaged. This is tightly regulated: a cell divides when it is told to, and when it becomes old or its DNA is damaged beyond repair, it destroys itself in a controlled process called apoptosis — programmed cell death.
Apoptosis is the body’s quality control. Cancer begins when that quality control fails.
The accelerator and the brakes
Think of cell division like driving a car. Two sets of genes control it.
| Oncogenes | Tumour suppressor genes | |
| Role | The accelerator — tell cells to grow and divide | The brakes — slow or stop cell division, trigger repair or apoptosis |
| What goes wrong | Become overactive — stuck on | Stop working — brakes fail |
| Example | RAS, MYC, HER2 | TP53, BRCA1, BRCA2, RB1 |
Cancer typically requires both failures — the accelerator jammed on and the brakes gone. A single mutation is rarely enough. This is why cancer risk rises steeply with age: mutations accumulate over decades until enough of them line up in the same cell.
Telomeres: why cancer cells do not die
At the end of every chromosome sits a protective cap called a telomere. Each time a normal cell divides, its telomeres get a little shorter. Once they are worn down, the cell can no longer divide and dies. It is a built-in division counter — a biological clock that limits how many times any cell can copy itself.
Cancer cells break the clock. They switch on an enzyme called telomerase, which rebuilds the telomeres after every division. The counter never runs out. The cell divides indefinitely. This is what oncologists mean when they describe cancer cells as “immortal” — and it is one of the defining hallmarks of cancer.
The four main causes of cancer
| Category | Examples | Modifiable? |
| Lifestyle | Tobacco, alcohol, obesity, poor diet, physical inactivity | Yes — largely |
| Environment | Air pollution, radiation, asbestos, industrial chemicals | Partly |
| Infection | HPV, Hepatitis B and C, H. pylori | Yes — vaccination and treatment |
| Heredity | Inherited BRCA1/2, Lynch syndrome and other mutations | No — but risk can be managed |
The critical point in that table is the right-hand column. Only 5 to 10% of cancers are inherited. The great majority are driven by factors that can be changed, avoided, vaccinated against, or caught early.
1. Lifestyle factors
Tobacco — the single largest preventable cause in India
Tobacco smoke contains thousands of chemicals, dozens of which are known carcinogens. It causes cancer of the lung, mouth, throat, larynx, oesophagus, bladder, kidney, pancreas, stomach and cervix — and contributes to several more.
India carries a particular burden here. Worldwide, lung cancer causes the most cancer deaths. In India, oral cancer does — a direct consequence of chewing tobacco, gutkha, khaini and betel quid (GLOBOCAN 2022, IARC). Smokeless tobacco is often assumed to be the safer option. It is not. It is why India has one of the highest oral cancer burdens in the world.
Quitting works, and it works at any age. Risk begins falling within a few years of stopping and continues to fall for decades.
Alcohol
Alcohol is classified by the IARC as a Group 1 carcinogen — the same category as tobacco and asbestos. It increases the risk of cancer of the mouth, throat, oesophagus, liver, breast and colorectum. The risk rises with the amount consumed, and there is no established “safe” threshold. Combining alcohol with tobacco multiplies the risk rather than simply adding to it.
Obesity, diet and physical inactivity
Excess body fat is linked to at least 13 cancers, including breast (post-menopausal), colorectal, endometrial, kidney, liver, pancreatic and oesophageal cancer. The mechanisms are chronic inflammation, elevated insulin and insulin-like growth factor, and altered sex hormone levels.
Diets high in processed meat and low in fibre, fruit and vegetables are associated with higher colorectal cancer risk. Processed meat is also classified by the IARC as a Group 1 carcinogen. Regular physical activity independently reduces the risk of several cancers.
Does stress cause cancer?
There is no good evidence that psychological stress directly causes cancer. This deserves a straight answer, because patients frequently blame themselves. Chronic stress can influence health indirectly — through inflammation, insulin resistance, poor sleep, and behaviours such as smoking, drinking and overeating — but the claim that stress itself causes tumours is not supported. Managing stress is worth doing for your wellbeing. It is not a cancer prevention strategy, and cancer is not your fault.
Sleep
Long-term night-shift work involving circadian disruption has been classified by the IARC as a probable carcinogen (Group 2A). The proposed mechanism involves suppression of melatonin, which plays a role in DNA repair and cell-cycle regulation. The evidence here is suggestive rather than settled — an occasional late night is not a cancer risk.
2. Environmental factors
Outdoor air pollution and fine particulate matter (PM2.5) are classified by the IARC as Group 1 carcinogens, causally linked to lung cancer. In Indian households, indoor air pollution from biomass cooking fuel is an additional and under-recognised exposure, particularly for women.
Occupational exposures matter too. Asbestos causes mesothelioma and lung cancer, often decades after exposure ends. Workers in construction, shipbreaking, textiles, dyeing and chemical manufacturing may face raised risk. If you work with industrial chemicals, workplace safety measures are not bureaucracy — they are cancer prevention.
3. Infections that cause cancer
Roughly one in six cancers worldwide is attributable to an infection — and this is the most preventable category of all, because vaccines and treatments exist.
| Infection | Cancer caused | Prevention |
| HPV (Human Papillomavirus) | Cervical, anal, oropharyngeal, penile, vulvar | HPV vaccine + Pap/HPV screening |
| Hepatitis B | Liver cancer | Hepatitis B vaccine |
| Hepatitis C | Liver cancer | Curable with antiviral treatment |
| Helicobacter pylori | Stomach cancer | Detectable and treatable with antibiotics |
Cervical cancer is the clearest example of what prevention can achieve. Virtually all cervical cancer is caused by persistent HPV infection, which means it is one of the very few cancers that can be almost entirely eliminated — through vaccination of girls before exposure, plus routine screening. It remains one of the leading cancers among Indian women largely because both interventions are under-used.
4. Hereditary factors
Only 5 to 10% of cancers are hereditary. The rest arise from mutations acquired during life, not inherited from a parent. A family history of cancer raises your risk but does not make cancer inevitable.
Features that suggest a genuine inherited cancer syndrome rather than coincidence:
- Cancer diagnosed at an unusually young age (for example, breast cancer under 45)
- Multiple close relatives with the same or related cancers
- The same person developing more than one primary cancer
- Cancers in both of a paired organ (both breasts, both kidneys)
- A rare cancer, such as male breast cancer or ovarian cancer
If several of these apply to your family, genetic counselling and testing may be appropriate. Knowing you carry a BRCA1, BRCA2 or Lynch syndrome mutation does not mean you will get cancer — it means you can start screening earlier, more often, and with a plan.
Can cancer be prevented?
Not every cancer, no. But a substantial share of cancers are preventable, and the actions are unglamorous and well-established:
- Stop all tobacco — smoked and smokeless. This is the single highest-impact action available to most Indians.
- Limit alcohol. Less is better; none is best.
- Get vaccinated — HPV vaccine for girls and boys, Hepatitis B vaccine.
- Maintain a healthy weight and stay physically active.
- Attend screening — mammography, Pap/HPV testing, and oral examination if you use tobacco.
- Act on persistent symptoms rather than waiting them out.
For what to watch for, read: Early Warning Signs of Cancer: 10 Symptoms Not to Ignore.
Frequently asked questions
If cancer runs in my family, will I get it?
No. Family history raises your risk, but most people with a family history of cancer never develop it — and most people who develop cancer have no family history at all. What a family history should change is your screening schedule, not your fatalism.
Does stress cause cancer?
There is no good evidence that psychological stress directly causes cancer. Chronic stress can affect health indirectly — through inflammation, sleep, and behaviours such as smoking and drinking — but stress is not a direct cause of tumours.
What is apoptosis?
Apoptosis is programmed cell death — the body’s controlled process for removing old or damaged cells. Cancer cells evade apoptosis, which allows them to survive and keep multiplying when they should have been destroyed.
Should everyone get genetic testing for cancer?
No. Genetic testing is recommended for people with a personal or family history suggesting an inherited cancer syndrome — early-onset cancers, multiple affected relatives, or certain rare cancers. For everyone else, it usually adds cost and anxiety without changing management.
Is chewing tobacco safer than smoking?
No. Smokeless tobacco — gutkha, khaini, betel quid, zarda — is a proven cause of oral, throat and oesophageal cancer. In India, oral cancer is the leading cause of cancer death, and smokeless tobacco is the principal reason.
Key takeaway
Cancer begins when the genes controlling cell growth and cell death stop working — the accelerator jams, the brakes fail, and the cell’s built-in clock is disabled. That damage builds up over years, and it is mostly not inherited. Only 5 to 10% of cancers are hereditary. The rest are shaped by things we can act on. Understanding the causes is the first step towards reducing your risk and recognising the warning signs early.
Consult a medical oncologist in Madurai
Dr. Deenadayalan T is a Consultant Medical Oncologist in Madurai, with expertise in cancer prevention, early diagnosis, precision oncology, targeted therapy and immunotherapy. If you have concerns about your family history or your cancer risk, book a consultation for individualised guidance.
Sources
- International Agency for Research on Cancer — Global Cancer Observatory (GLOBOCAN)
- IARC — Agents classified by the IARC Monographs
- World Health Organization — Cancer fact sheet
- National Cancer Institute — Risk factors for cancer
Medically reviewed by Dr. Deenadayalan T, MD, DM (Medical Oncology). Last reviewed: [15 July 2026]. This article is for general information only and is not a substitute for professional medical advice, diagnosis or treatment. Consult a qualified doctor about your individual risk.