By William C. Phelps, PhD
How did you feel the last time someone sneezed in the elevator? Whether it is the common cold or the seasonal flu, we know some illnesses are caused by infections with viruses or bacteria. But what if cancer could be caused by an infection?
Some cancers caused by viruses and bacteria
Although it is not widely realized, 15%-20% of cancers around the world are caused by infectious agents - viruses or bacteria. Fortunately for all of us, the infectious agents linked to cancer are not easily spread from person to person like the common cold virus. It turns out, even when many of these viruses and bacteria infect people, only a small subset will go on to develop cancer. In most cases, we still do not understand why certain people develop cancer and others do not - even though they were also infected.
A number of different types of infections can cause cancer. The Epstein-Barr virus can cause lymphomas (cancers of the lymphatic system) and nasopharyngeal cancer. Kaposi sarcoma virus causes a form of skin cancer in patients with HIV, the virus that causes AIDS, but only after HIV has already damaged the patient's immune system.
We also know today that one of the important causes of stomach cancer is long-term bacterial infection with Helicobacter pylori. Helicobacter is thought to be spread through contaminated food or water, infecting the stomach, where it is a major cause of ulcers. Chronic ulcers as a result of the Helicobacter bacteria greatly increase the risk for developing stomach cancer. Because of this, a half-century ago, stomach cancer was the second most common form of cancer in the world. Today, stomach cancer does not occur nearly as often, and that may be due to wider use of refrigeration, availability of clean water, and better hygiene, which limit the spread of bacteria.
Perhaps the most prevalent cancer-causing virus is human papillomavirus (HPV), which has been in the news a lot during the past few years. HPV causes cervical and anal cancer, and has recently been linked to about half of oral cancers. HPV is passed from person to person through close contact, most often through sex.
For more than 50 years, women have been effectively screened for cervical cancer with a Pap test (or smear) which detects abnormal-looking cells from the cervix. It wasn't until the 1980s that scientists determined that those odd-looking cells were actually infected with a virus. That Nobel Prize-winning discovery was the foundation for the development of new diagnostic tests (HPV DNA Test) that help doctors monitor patients, and very effective vaccines (Gardasil and Cervarix) that can prevent cervical cancer by preventing HPV infection.
Vaccinations save lives
What many don't realize is that the HPV vaccine became the second vaccine available to prevent cancer.
Hepatitis viruses (HBV and HCV) can cause liver cancer, A vaccine for Hepatitis B has been available since the 1980s and has been routinely included in infant vaccines in the United States since the 1990s. The vaccine is very effective in preventing HBV infection. In Canada, for instance, the rate of HBV infection had fallen from 1.7 cases per 100,000 people to 0 per 100,000 just 10 years after children started being vaccinated.
In fact, there is probably no easier or more effective form of cancer prevention than vaccination against HBV to prevent liver cancer or HPV to prevent cervical cancer. In spite of that, only 1 out of 3 people in the US has been vaccinated for HBV, and fewer than 1 in 5 women between the ages of 19 and 26 has received HPV vaccinations. We have to do better as a society since preventing the cancer in the first place saves lives and money.
Another role for bacteria
Not all infections are harmful, though. Research over the past decade has revealed that helpful microorganisms like bacteria in people's intestines help digest food and play an important role in the health of the cells that line the intestinal tract. When this relationship is disrupted, however, people can develop ulcerative colitis and Crohn's disease, which gives them a higher risk for the development of colon cancer. Even if you don't have one of those conditions, what you eat can also affect the normal balance of intestinal microorganisms. Over the long run, this could impact the risk for developing colon cancer. Researchers are still teasing out the relationship between diet, intestinal bacteria, and cancer risk.
The search goes on
Today the search goes on for new infectious agents which may play a role in causing cancer. Only 4 years ago, a previously unknown virus, Merkel Cell Polyomavirus, was discovered in association with a rare form of skin cancer. In the past few years, viruses have been found in lung cancer and brain tumors, and bacteria and protozoans have been associated with prostate cancer. We're continuing to look into whether and how viruses affect those and other cancers. Infectious diseases have been studied for hundreds of years, yet today there is still much that we do not understand about how they infect and affect humans.
For the future, as we focus on exciting new targeted therapies and personalized medicine to treat cancer, the role of viruses and bacteria in cancer is something to consider.
Dr. Phelps is director of preclinical and translational cancer research for the American Cancer Society