The cause of most non-Hodgkin lymphomas is unknown. However, lymphomas are more likely to develop in people who have taken drugs to prevent rejection of an organ transplant or who have lowered immunity, for example due to HIV or Aids. Despite the increased risk, NHL is still uncommon in people with these conditions.

Non-Hodgkin lymphoma is also slightly more common in people who have been treated for cancer before. Some chemotherapy drugs and radiotherapy treatments can increase a person’s risk of developing NHL many years later. The risk is very small, however, compared to the benefit of having the treatment in the first place.

Certain viruses, such as the Epstein-Barr virus, which causes glandular fever, or the human T-cell lymphoma virus 1 (HTLV1) can contribute to the development of lymphomas. However, like other cancers, lymphomas themselves are not infectious and cannot be passed on to other people.

One type of lymphoma, which usually affects the stomach – MALT lymphoma – is known to be caused by a type of bacterial infection known as helicobacter pylori.

People who have coeliac disease (an allergy to gluten which can cause inflammation of the small bowel) have a slightly increased risk of developing B-cell NHL or a rare type of T-cell lymphoma called enteropathy type T-cell lymphoma (ETTL).