Haemophagocytic lymphohistiocytosis (HLH) is a rare life-threatening blood disease which causes severe inflammation with symptoms similar to severe sepsis. It is hard to diagnose. The most common cause of HLH in adults is lymphoma (blood cancer). Outcomes for adults with HLH and cancer are serious, and most die after days or weeks because they have been diagnosed or treated too late. It is likely that many cases where patients died of HLH with no underlying cause actually had cancer. Recently it has been found that patients with certain types of lymphoma have DNA which comes directly from their cancer (circulating tumour DNA; ctDNA). Aggressive lymphomas release a lot of ctDNA which can be detected in the blood of patients. This study will look for ctDNA in patients with HLH, and see if it is possible to use it to diagnose lymphoma earlier. Patients will provide a small additional blood sample for analysis. Diagnosing lymphoma more rapidly would mean more people could get the correct treatment for the lymphoma which has caused their HLH. They could receive the correct treatment sooner. Earlier diagnosis and treatment could improve survival for these patients.