In a context of viral gastroenteritis, we report an unusual case of sudden death in an 8-year-old child. The only macroscopic abnormality observed in the autopsy was a diffuse mesenteric adenitis. Organ samples were taken for histopathological examination and a diffuse lymphocytic infiltration was observed. A sinusoidal histiocytic hyperplasia was found in the lymph nodes. Microscopic examination of the lungs and the lymph nodes revealed haemophagocytic lesions (lymphocytes within the cytoplasm of histiocytic macrophages). Immunohistochemical studies demonstrated that the histiocytes were CD68+ PS100- CD1A-. Following this microscopic examination macrophage activation or haemophagocytic syndrome was diagnosed. The syndrome is a distinct clinical entity characterised by fever, pancytopaenia, splenomegaly, and haemophagocytosis in the bone marrow, liver and lymph nodes. It is a clinical entity that is very difficult to diagnose due to the lack of specific clinical signs. It is generally a complication of an infectious process, an aggravation of an auto-immune disease or a complication of a neoplastic process. The physiopathology involves a disregulation of T lymphocytes and particularly T helper lymphocytes. To make this diagnosis the anatomopathological examination must be performed by an experienced practitioner. The presence of a lymphocyte infiltrate of macrophagic histiocytes in myeloid organs and especially positive CD68+ immune markers are the anatomopathological proofs of diagnosis. The autopsy examination must be carefully performed and include systematic sample harvesting for anatomopathological examination. The results of all these examinations taken together allow the diagnosis of haemophagocytic syndrome to be finally made.