Infectious agents and cancer

Abstract

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. In the United States and other developed countries, less than 10% of all cancers are thought to be linked to infectious agents. But in developing countries, infections can account for as many as 1 in 4 (25%) of all cancers. Infections might raise a person’s risk of cancer in different ways. Some infections can cause long-term inflammation, suppress a person’s immune system, or directly affect a cell’s DNA. 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. The infections that influence cancer risk may be contagious, but cancer itself is not. A healthy person cannot “catch” cancer from someone who has it.