A 47-year-old patient came to see a doctor on the 7th day of disease. The disease is acute: after the chill body temperature rose to 40°C and lasted up to 7 hours, then dropped abruptly, which caused profuse sweat. There were three such attacks occurring once every other day. Two days ago the patient arrived from Africa. Objectively: pale skin, subicteric sclera, significantly enlarged liver and spleen. What is the cause of fever attacks in this disease?