My Niece Is About To Be 2 And She Has

My niece is about to be 2 and she has had a fever on and off for about 3 weeks and it escalates really fast and can get up to 105. She's lethargic and we've tried antivirals and antibiotics and haven't worked yet. She has elevated leukocytes and swollen lymph nodes and she is bruising easily. What do you think could be causing this?