Suggest Cause And Remedy For Fever And Headaches

Good evening. I have a male friend, 21 years old, 5 7 in height and weight is 125 to 130 lbs. He had Dengue last January. He also had a brain tumor and it was removed last February. It was malignant. No chemo therapy is being done. He has been taking medicines everyday, glucose every night and pain killers for headache especially now that it is summer in his country. He is experiencing fever and headache for 4 days now. His fever is on and off and it usually comes at nights only. He went to the doctor today but the doctor said, nothing serious. But there are times when he cannot sit on a chair. I also remember when he fell off his chair when he had Dengue (he also had brain tumor at that time but not treated yet). May I ask, what are the possible causes of his fever? Thank you so much.