Why Is My Brother Still Experiencing Recurring Fever After 12 Days Despite His Platelet Count Improving?

Hy Doctor My brother suffering from fever From last 12 days. His covid report is negative. Five days ago his platelets fall down to 64k and TLC was 32000 After that we go to doctor he gave him Injection to increase his platelets. After 2 days his platelets count increase to 69k and TLC was 2800 Today we do cbc now his platelets increase to 93k and his Tlc was 3200 But his fever not going down He has fever of 99-100 degree In last 12 day After one or 2 days he is getting fever again and again After 1 hours his fever gone Can you pls consult something on this.