Non-Hodgkin lymphoma is caused by a change (mutation) in the DNA of a type of white blood cell called lymphocytes, although the exact reason why this happens isn't known.
DNA gives cells a basic set of instructions, such as when to grow and reproduce.
The mutation in the DNA changes these instructions, so the cells keep growing. This causes them to multiply uncontrollably.
The abnormal lymphocytes usually begin to multiply in 1 or more lymph nodes in a particular area of the body, such as your neck, armpit or groin.
Over time, it's possible for the abnormal lymphocytes to spread into other parts of your body, such as your:
But in some cases non-Hodgkin lymphoma starts growing in an organ, rather than starting in a lymph node and spreading to an organ.
While the cause of the initial mutation that triggers non-Hodgkin lymphoma is unknown, a number of factors can increase your risk of developing the condition.
These include having:
Non-Hodgkin lymphoma isn't infectious and isn't thought to run in families, although your risk may be slightly increased if a first-degree relative (such as a parent or sibling) has had lymphoma.
It can occur at any age, but a third cases are diagnosed in people over 75 and the condition is slightly more common in men than women.