June 20, 2008
Everyone knows that smoking is the No. 1 cause of cancer. Can you name No. 2?
People tend to think that cancer is caused only by unhealthful habits, such as smoking, or environmental factors, including exposure to asbestos or excessive sunlight. They don't realize that cancer often is triggered by an infectious disease.
At least 25 percent of malignancies are caused by viruses, bacteria and parasites. After smoking, infection is the leading cause of cancer. Although millions of Americans are infected with cancer-causing organisms at some time during their lives, most don't develop cancer as a result. There are additional risk factors that work in tandem with infectious microbes to trigger the biological changes that lead to cancer.
Viruses are the main cancer-causing organisms, followed by bacteria and parasites. The primary ways these organisms cause cancer include the following:
Viruses can't replicate on their own. When viruses enter your body, they inject their own genetic material into your cells and take over the cells' inner workings.
Certain viral genes, known as oncogenes, cause cells to divide more rapidly. Rapid cell division increases the odds of genetic "mistakes" that can lead to cancer. Viruses also inhibit our body's natural ability to destroy damaged cells, which may otherwise continue to grow and divide in ways that make us more vulnerable to cancer.
Some organisms, such as those that cause stomach and liver cancer, irritate tissues and trigger persistent inflammation. Inflammation causes cells to divide at a faster rate than normal, increasing the likelihood that they will mutate and undergo the changes that can lead to cancer.
Approximately 30 to 40 percent of Americans are infected with Helicobacter pylori, a screw-shaped bacterium that burrows into the stomach lining and causes chronic inflammation. About 20 percent of these people will eventually develop ulcers and another 5 percent will develop stomach cancer.
Infection with H. pylori is a very strong risk factor for cancer, presumably because the bacterium causes inflammation and cell proliferation. More than 80 percent of stomach cancer cases are caused by this bacterium: Infection increases your risk of developing stomach cancer by at least eightfold.
Self-defense: Ulcer patients are routinely tested for H. pylori and treated with antibiotics if infection is present. Once the bacterium is eliminated, the risk for ulcers drops significantly. It is not yet known if treating bacteria will help prevent stomach cancer. Patients with a family history of stomach cancer should talk to their doctors about getting tested for H. pylori.
It's also helpful to eat a nutritious diet that is rich in fruits and vegetables, and low in salt and food preservatives known as nitrates. Such a diet may reduce cancer risk.
The Epstein-Barr virus (EBV) causes infectious mononucleosis, which leads to extreme fatigue and other flulike symptoms. EBV is found in the tumors of a significant number of patients with Hodgkin's disease (a form of lymphoma that strikes most often between the ages of 15 and 35 and after age 55).
The risk that an individual who has had mononucleosis will go on to develop Hodgkin's disease or non-Hodgkin's lymphoma, which is also associated with the EBV virus, is still very low. The main risk for non-Hodgkin's lymphoma appears to be in patients who have severely compromised immune systems-for example, those who have undergone transplant surgery or who are taking immune-suppressive drugs.
Self-defense: Transmission of EBV is impossible to prevent because many healthy people can carry and spread the virus for life. People who receive transplants and immune-suppressing drugs should ask their doctors about symptoms of EBV-related malignancies. Decreasing immunosuppression can often reverse lymphoma when caught early.
The hepatitis B virus is spread by contact with body fluids of an infected person, including blood, saliva, vaginal secretions and semen. People who have sex with infected partners and drug users who share needles are at greatest risk for contracting this virus.
Most cases of hepatitis B are acute, lasting six months or less. This form of hepatitis is not linked to cancer. However, the chronic form of hepatitis B, which is almost always acquired in childhood and lasts for more than six months, greatly increases the risk for cirrhosis (destruction of normal liver tissue) as well as liver cancer.
Self-defense: All newborns are now given the hepatitis B vaccine, which is also recommended for children aged 18 years or younger who weren't previously vaccinated. Adults don't require the vaccine for cancer prevention, but it is recommended for people in high-risk groups because it can reduce the risk of long-term liver disease.
People who should get the vaccine include health-care workers, those who are sexually active or have household contact with people who may have hepatitis B, and dialysis patients.
Most people with the hepatitis C virus were infected by tainted blood transfusions prior to 1992, when blood-screening tests first became available. Hepatitis C also can be transmitted by sex with an infected partner, shared contaminated hypodermic needles, and nonsterile tattoo or body-piercing procedures.
About 5 percent of patients with hepatitis C will develop liver cancer. A much higher percentage will develop cirrhosis or other chronic liver diseases-usually decades after the initial exposure.
Self-defense: In addition to the high-risk practices mentioned above, do not share razors, toothbrushes or nail clippers in households with an infected person. Patients who received a blood transfusion prior to 1992 or who engage in high-risk practices should be tested.
Julie Parsonnet, MD, is a professor of infectious diseases and geographic medicine, and of health research and policy at Stanford University Medical Center, and is an expert on infection as a cause of chronic disease.
Reprinted with the permission of Bottom Line, Boardroom Inc., 281 Tresser Blvd., 8th Floor, Stamford, CT 06901. BottomLineSecrets.com.
Stanford Medicine integrates research, medical education and health care at its three institutions - Stanford School of Medicine, Stanford Health Care, and Stanford Children's Health. For more information, please visit the Office of Communications website at http://mednews.stanford.edu.