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Hodgkin lymphoma is caused by a change (mutation) in the DNA of a type of white blood cell called B lymphocytes. The exact reason why this happens isn't known.

The DNA gives the 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, causing them to multiply uncontrollably.

The abnormal lymphocytes usually begin to multiply in one or more lymph nodes in a particular area of the body, such as your neck or groin. Over time, it's possible for the abnormal lymphocytes to spread into other parts of your body, such as your:

• bone marrow
• spleen
• liver
• skin
• lungs

While the cause of the initial mutation that triggers Hodgkin lymphoma is unknown, a number of factors can increase your risk of developing the condition. These include:

• age and gender – anyone can get Hodgkin lymphoma but it's more common in people aged 20 to 40 or over 75; it also affects slightly more men than women
• having a medical condition that weakens your immune system, such as HIV
• having medical treatment that weakens your immune system – for example, taking medicine to suppress your immune system after an organ transplant
• being previously exposed to the Epstein-Barr virus (EBV) – a common virus that causes glandular fever
• having previously had non-Hodgkin lymphoma, possibly because of treatment with chemotherapy or radiotherapy
• being very overweight (obese) – this may be more of a risk factor in women than men
• smoking

Hodgkin lymphoma isn't infectious and isn't thought to run in families. Although your risk is increased if a first-degree relative (parent, sibling or child) has had lymphoma, it's not clear if this is because of an inherited genetic fault or lifestyle factors.