You’ve probably heard of cancer before, and you may know that it’s one of the deadliest diseases out there, killing around 8 million people every year worldwide. But did you know that only 5% of those deaths are caused by cancer itself? That’s right—of all the people who die from cancer, 95% of them actually died from other causes! But what are the causes of cancer?

The causes of cancer are varied and complex, but fortunately, there are many preventative actions you can take to reduce your risk of getting cancer or to slow its progression should you already have it.

Cancer isn’t just one disease; it’s an umbrella term that encompasses nearly 100 diseases and conditions, each with its own causes and symptoms. Understanding what causes cancer can help you better understand how to prevent and treat the disease, whether it be in yourself or someone else.

Read on to find out what makes someone more likely to get cancer than others and how you can protect yourself from the disease as much as possible.



It’s easy to overlook smoking as a cause of cancer since it’s not typically considered a disease. However, tobacco smoke contains thousands of toxic chemicals that are carcinogenic and cause lung cancer. In fact, smoking is responsible for nearly half (45%) of all lung cancers in men and 70% in women. Lung cancer is also more common among African-Americans and people who live in poverty.

Smoking cigarettes is, by far, one of the most preventable causes of cancer. If you know someone who smokes, help them kick their habit today. If you have never smoked but are considering it, know that there are many resources available to help you quit for good.


Exposure to radiation – from diagnostic x-rays, CT scans, dental x-rays, flying in airplanes, etc. – could theoretically increase one’s cancer risk. Dr. Ben Lynch from California Pacific Medical Center reports that it is not yet known whether or not repeated exposure to low levels of radiation over time can cause cancer.

While radiation is more commonly associated with causes of cancer like skin cancer, it can also cause leukemia and other forms. Exposure to radiation increases your risk for cancer by damaging cells in your body, which might then become cancerous. But be sure to limit your number of radiological procedures if you are concerned about long-term health risks.

Some Medical Treatments Can Cause Cancer

While medical treatments are often necessary to combat cancer, some may have a negative effect on an otherwise healthy patient. The medical procedure itself may cause cancer or increase its risk, or it may cause another health problem that later turns into cancer. This can be seen in chemotherapy—particularly older methods of chemotherapy that utilize more toxic chemicals—which can lead to a higher likelihood of blood cell mutations and, ultimately, leukemia.

Recently, it was also discovered that a common treatment for irritable bowel syndrome (dubbed the colonic) may be associated with an increased risk of developing colorectal cancer. The National Institutes of Health recommends you contact your doctor if you have questions about any possible link between medicine or procedure and your risk for developing cancer.


As you get older, your cells replicate more slowly and contain more DNA damage. This means they are more likely to mutate into cancerous cells. Some studies suggest that as we age, our immune systems become less adept at killing off mutated cells before they turn malignant. Meanwhile, as our organs age, they produce less energy than they did when we were younger (due to a steady decline in mitochondrial function) – which can create an environment in which mutations can thrive and grow into full-blown tumors.

The aging process alone is unlikely to cause cancer, but aging does appear to play a role in some types of cancer. Research has found that people with Down syndrome are almost twice as likely to develop leukemia or certain other cancers.

Genetic Predisposition

One cause of cancer is a genetic predisposition; if your parents had breast cancer, for example, you’re at a greater risk. Scientists have known for a long time that DNA can make people more susceptible to cancer. For example, if you have a family history of breast cancer, your risk is higher than most women. This is because there’s usually a genetic predisposition to certain cancers. If one or more close relatives on your mother’s side were diagnosed with breast cancer before age 50, you may be at greater risk for developing it as well.

While not all genetic changes directly cause cancer, some people may be genetically more susceptible to developing it. To date, scientists have identified more than three dozen genes associated with a higher risk for certain types of cancer. These gene mutations are what experts call cancer predisposition syndromes—predisposing factors or vulnerabilities that boost your chance of developing cancer but that don’t necessarily mean you will develop it; there is also a long list of known non-genetic causes of cancer.

Mother’s Diseases during Pregnancy or Childhood Infections

A mother’s disease during pregnancy or when she was a child can cause cancer. Some types of birth defects, such as cleft palate, which is when a person’s mouth doesn’t form correctly during development, are caused by specific infections. And some diseases that women have before they get pregnant—such as breast cancer or cervical cancer—can increase their chances of having a baby with certain birth defects.

Children whose mothers have certain infections during pregnancy or childhood can also develop cancer. For example, if you catch German measles during your first trimester, your child can be born with leukemia. If you’re pregnant and get chickenpox, your baby could develop SSPE (subacute sclerosing panencephalitis), a type of encephalitis that causes brain damage and eventually death.

Family History

If you have a family history of cancer, it’s likely that you will inherit at least some part of your risk for developing cancer. If your father had colon cancer or lung cancer, for example, you may have a genetic predisposition to these forms of the disease.

While lifestyle choices have a role in cancer, so does genetics. If you have a family history of cancer, it’s important to talk to your doctor about ways to decrease your risk. According to The American Institute for Cancer Research (AICR), here are some key lifestyle changes that could reduce your risk of quitting smoking; exercising regularly; maintaining a healthy weight; eating more plant-based foods and less red meat; limiting alcohol consumption to one drink per day or less. AICR also recommends scheduling regular physical exams and screening tests as recommended by your doctor based on age, gender, and health history.

Poor Diet and Lifestyle


For years, people have known that smoking, excessive drinking, and radiation are causes of cancer. But what about diet and lifestyle? Scientists are increasingly recognizing that our diets and lifestyles are major players in causing cancer. A diet high in red meat and low in fiber has been linked to colon cancer; pesticides used on crops have been linked to non-Hodgkin’s lymphoma; getting less than 6 hours of sleep each night has been linked to breast cancer.

Exposure to Carcinogens

Carcinogens, inorganic and organic substances that cause cancer, are everywhere. Many carcinogens cannot be avoided altogether. For example, drinking milk (which is a rich source of calcium) may expose us to carcinogenic hormones that cows are exposed to through synthetic growth hormones given to them by farmers. Exposure can occur during medical treatment as well; many chemotherapy drugs kill healthy cells along with cancerous ones.

There are a number of carcinogens (chemicals and physical agents) that cause cancer. These include things like asbestos, tobacco smoke, carbon monoxide, and radon gas. Carcinogens can either be inhaled or they can penetrate your skin. If you’re exposed to high levels of these chemicals over a period of time, it will likely lead to cancer. This doesn’t mean you can avoid all possible carcinogens—but doing so might help lower your risk for developing certain cancers later in life.

Viruses, Bacteria, Fungi, and Parasites

There are many types of bacteria and viruses that can cause cancer in humans. In addition, fungi and parasites have been linked to certain types of cancer. Viruses directly cause approximately 15% of all cancers in humans. Hepatitis B, Human papillomavirus (HPV), Epstein-Barr virus (EBV), and Kaposi’s sarcoma-associated herpesvirus are just a few examples.


Cholesterol is one cause of cancer that you can prevent by cutting back on unhealthy fats, like those found in red meat, dairy products, and other processed foods. Limiting your intake of these food groups can greatly reduce your cholesterol levels.

Your body needs cholesterol to produce certain hormones, vitamin D, and proteins. But if you eat a diet high in saturated fats (meat) or trans-fats (processed foods), your body makes more cholesterol than it needs. This can lead to higher levels of low-density lipoprotein, or LDL. When you have a lot of LDL in your bloodstream it causes blockages that will eventually damage your heart and liver, possibly causing cancer.


Obesity is a major contributing factor to cancer risk, as it increases your chances of getting inflammatory breast cancer, colorectal cancer, postmenopausal breast cancer, and cancers that start in fat cells. The most recent statistics say that over one-third of American adults are obese. Overweight children are also at an increased risk for obesity in adulthood.

One study found that obese people had a 58 percent higher risk for cancer than normal-weight individuals, and being overweight has been linked to an increased risk of breast, endometrial, colorectal, liver, pancreatic, esophageal, and kidney cancers. Luckily there are plenty of things you can do to shed pounds safely—and keep them off for good.


One word can hold so much weight. Pollution is a complex environmental issue, but it can be broken down into simple terms. According to Environmental Protection Agency (EPA), Pollution means anything humans introduce into our environment that negatively affects plants, animals, or human health. Pollution comes in many forms such as air pollution, water pollution, noise pollution, and toxic waste.

Tumors have been found to form in areas that have high levels of environmental pollution, according to a 2014 study published in Environmental Pollution. It’s thought that certain chemicals and toxins cause DNA damage, which can lead to cancerous tumors. For instance, polycyclic aromatic hydrocarbons (PAHs) — created when coal is burned — are a known carcinogen.


The sun is a major source of ultraviolet (UV) radiation, which can cause mutations in our DNA. A little sunshine is good for us—vitamin D is produced in our skin with exposure to UVB rays—but too much can cause serious problems. People who burn easily or have freckles are more susceptible to skin cancer due to their lack of melanin, which acts as a natural sunscreen.

Researchers have known for some time that exposure to ultraviolet radiation increases one’s risk of developing skin cancer. More recently, however, scientists have been discovering links between exposure to UV light and other forms of cancer as well. To lower your chances of developing any type of cancer, avoid excessive sun exposure—especially when you’re younger.


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