Hemophagocytic lymphohistiocytosis (HLH) is a rare and potentially fatal condition that occurs when certain white blood cells build up in and damage organs, including the bone marrow, liver, and spleen, and destroy other blood cells. HLH can occur in both children and adults, and there are two types: familial and acquired. Familial HLH accounts for about 25% of cases and is inherited from parents who are genetic carriers of the condition. Acquired HLH can be caused by a number of conditions, including viral infections, autoimmune diseases, and cancer. Symptoms of HLH can include fever, rash, enlarged liver and spleen, pancytopenia (low counts of all blood types), and neurological symptoms such as confusion and seizures. HLH can only be diagnosed with proper blood tests in the appropriate clinical setting. Healthcare providers diagnose HLH based on symptoms, physical exam findings, and several lab tests, including a complete blood count and genetic testing. Early diagnosis and treatment are key, and treatment options include chemotherapy, immunotherapy, and stem cell transplant.