Can Infections Cause Cancer?

Since the start of the 20th century, it’s been known that certain infections play a role in cancer in animals. More recently, infections with certain viruses, bacteria, and parasites have been recognized as risk factors for several types of cancer in humans.

Worldwide, infections are linked to about 15% to 20% of cancers. This percentage is even higher in developing countries, but it is lower in the United States and other developed countries. This is partly because certain infections are more common in developing countries, and partly because some other risk factors for cancer, such as obesity, are more common in developed countries.

Infections can raise a person’s risk of cancer in different ways. For example:

• Some viruses directly affect the genes inside cells that control their growth. These viruses can insert their own genes into the cell, causing the cell to grow out of control.
• Some infections can cause long-term inflammation in a part of the body. This can lead to changes in the affected cells and in nearby immune cells, which can eventually lead to cancer.
• Some types of infections can suppress a person’s immune system, which normally helps protect the body from some cancers.

Any of these changes might lead to a higher risk of cancer.

Even though the infections described here can raise a person’s risk of certain types of cancer, most people with these infections never develop cancer. The risk of developing cancer is also influenced by other factors. For example, infection with Helicobacter pylori (H pylori) bacteria might increase your risk of stomach cancer, but what you eat, whether or not you smoke, and other factors also affect your risk.

Many of the infections that influence cancer risk can be passed from person to person, but cancer itself cannot. A healthy person can’t “catch” cancer from someone who has it.