There is no single cause of non-Hodgkin’s lymphoma (NHL). Researchers have identified a number of factors that appear to place a person at higher risk for developing the disease. These risk factors include:

• Age. Most cases of non-Hodgkin’s lymphoma occur in people who are in their 60s. Age is the greatest risk factor for this form of cancer.
  
  
• Congenital immune deficiency diseases. Being born with an abnormal immune system increases a person’s risk of developing NHL during childhood or young adulthood.
  
  
• Obesity. In the past, obesity was linked to a higher risk of NHL. Recent studies from the National Cancer Institute, however, have found that obesity may not increase a person’s chances of developing NHL or other malignant lymphoma.
  
  
• Radiation. Patients who have received radiation therapy as treatment for other forms of cancer have a slightly increased risk of developing NHL later in life. This risk is greatest for those patients treated with both radiation therapy and chemotherapy. In addition, people who have survived (or lived in close proximity to) nuclear blasts and accidents have an increased risk of developing NHL.
  
  
• Exposure to certain chemicals. Some studies have suggested a link between certain chemicals, including benzene and particular herbicides and insecticides, and an increased risk of developing NHL. Recent studies have linked exposure to PCBs (polychlorinated biphenyls) and compounds from the pesticide DDT to an increased risk for developing NHL. However, research is continuing to verify this connection.
  
  
• Tobacco use. Recent studies suggest that tobacco use increases the risk of developing NHL. Previous research did not link tobacco use to NHL in either women or men. However, more controlled studies in recent years have found an increased risk, particularly with heavy smokers.
  
  
• Chemotherapy drugs. Certain chemotherapy drugs used to treat other types of cancer can increase a person’s risk of developing NHL later in life. However, a direct cause and effect relationship has not yet been proven.
  
  
• Immunosuppressant drugs. Patients such as kidney transplant recipients being treated with immunosuppressant drugs (drugs that weaken the immune system) are at an increased risk of developing NHL. The exact increase in risk is dependent on the type of medication and the dose.
  
  
• Human immunodeficiency virus (HIV). Being infected with HIV increases a person’s risk of developing certain types of NHL.
  
  
• Human T-cell leukemia/lymphoma virus (HTLV-1). Being infected with HTLV–1 increases a person’s risk of developing certain types of NHL. Belonging to the same virus family as HIV, this virus is most common in the Caribbean and some parts of Japan. It is far less common in the United States, where it accounts for less than 1 percent of lymphomas.
  
  
• Epstein-Barr virus (EBV). EBV is the virus that causes mononucleosis. In certain areas of Africa, infection with the EBV is considered an important risk factor for Burkitt’s lymphoma. EBV is a risk factor for lymphomas in people in other countries as well, particularly those infected with HIV. In addition, infection with other viruses, such as hepatitis C, also may increase the risk of NHL.
  
  
• Infection with the parasite that causes malaria. In certain areas of Africa, infection with the parasite that causes malaria is considered an important risk factor for Burkitt’s lymphoma.
  
  
• Helicobacter pylori. Known to cause stomach ulcers, this type of bacteria can also trigger some lymphomas of the stomach.

Although NHL is associated with these risk factors, the majority of patients with the disease have no known risk factors.