Recently, science has begun to recognize the cancer risk posed by certain viruses, bacteria and parasites in humans. Only about 10 percent of cancers in the United States are linked to these agents, but they may contribute to as much as 20 percent of cancers in developing countries, according to the American Cancer Society (ACS).  Most people who have infections never develop cancer and even though infections are often contagious, the potential cancer is not. Infectious agents associated with cancer include:

• Viruses. Consist of a small group of genes surrounded by a protein coating. They enter a cell and use the cell machinery to produce more viruses. In some cases, the virus inserts its DNA or RNA into the host cell. This can affect the host cell’s genes and set the stage for it to become cancerous. Viruses known to sometimes cause cancer include:
  
  
• Human T-cell leukemia/lymphoma virus-1 (HTLV-1). Associated with a type of lymphocytic leukemia and non-Hodgkin’s lymphoma called adult T-cell leukemia/lymphoma (ATL). It is found mostly in southern Japan, the Caribbean, Central Africa, parts of South America and in some immigrant populations in the southeastern United States.
  
  
• Human papillomaviruses (HPVs). A group of more than 100 related viruses that manifest as warts on the skin, mouth, genital organs or larynx. Certain types of HPV are the primary cause of cervical cancer. Others contribute to cancers of the penis, anus, vagina and vulva. In 2006, the Food and Drug Adminstration approved the first vaccine for several types of HPV linked to cervical cancer. Recent studies have suggested that testing for past exposure to these types of HPV eventually may be able to replace Pap tests for cervical cancer screening in women.
  
  
• Human herpes virus 8 (HHV-8). Also known as Kaposi sarcoma–associated herpes virus (KSHV), it is found in virtually all tumors in patients who have Kaposi sarcoma. This is a rare, slow-progressing cancer that causes tumors just beneath the skin. Recent findings also indicate that it may be a factor in the development of plasma cell tumors such as multiple myeloma.
  
  
• Epstein-Barr virus (EBV). Another herpes virus probably best known for causing infectious mononucleosis, also known as "mono." It infects and resides in white blood cells and increases the risk of nasopharyngeal cancer, stomach cancer, Hodgkin’s lymphoma and some types of non-Hodgkin’s lymphoma. Research has suggested it also may play a role in aggressive forms of breast cancer. If an individual is infected with EBV, it usually occurs before the age of 20. The great majority of people infected with EBV never develop cancer. Cancers associated with this virus are more common in Africa and parts of Southeast Asia.
  
  
• Human immunodeficiency virus (HIV). HIV, the virus that causes AIDS, it is not thought to directly contribute to the development of cancers, but does increase the risk of some types of cancer such as Kaposi sarcoma, Hodgkin’s lymphoma, certain types of non-Hodgkin’s lymphoma, testicular cancer and penile cancer.
  
  
• Hepatitis B virus (HBV) and hepatitis C virus (HCV). These viruses cause a type of liver infection known as viral hepatitis that can become chronic and increase the risk of hepatocellular carcinoma, the chief form of liver cancer. Between 25 percent and 50 percent of hepatocellular carcinomas are related to these infections. HCV may also be a factor in some types of non-Hodgkin’s lymphomas.
  
  
• Bacteria. Microscopic one-celled organisms that are self-sufficient and that multiply by subdivision. In some cases, bacteria are not harmful to the body, and some are actually helpful. However, infectious bacteria can cause illness. Bacteria associated with cancer include:
  
  
• Helicobacter pylori (H. pylori). This bacterium can cause chronic atrophic gastritis, inflammation and damage to the inner layer of the stomach. This is a change in the stomach lining that may become precancerous. Patients with the most common form of stomach cancer (adenocarcinoma) have a higher rate of infection with the bacteria than people without the cancer. H. pylori infection also has been associated with some stomach lymphomas.
  
  
• Chlamydia trachomatis. Bacteria that infects the female reproductive system that may raise the risk for cervical cancer.
  
  
• Parasites. Certain parasitic worms that live in the human body can raise the risk of cancer. These organisms are not found in the United States, but are a potential danger to people who live or travel in other regions of the world. This parasites include:
  
  
• Opisthorchis viverrini and Clonorchis sinensis. These are types of flatworms known as liver flukes that have been associated with increased risk of cancer of the bile ducts. These infections occur almost exclusively in East Asia.
  
  
• Schistosoma haematobium. Infection is known as schistosomiasis, and it has been associated with cancer of the urinary bladder. The parasite is found in developing countries in Africa and Asia.