Thanks to HCM I can understand your concern there are many etiological factors for pyrexia associated with rigors it may be a bacterial,viral or fungal infections. so plz check once all basic investigations like CBP,ESR,LFT,RFT.Widal test for typhoid ,Rapid malaria test .blood culture ,cue,urine culture generally in old age .inflammation over joints leads to high grade fever with chills &rigors as you said serum markers are raised ,plz continue the antibiotics,antiinflammatory drugs antipyretics and if need steroid of low dose required . don t worry it will subside , take care of dehydration of body drink plenty of liquids and fruits . onemore thing plz undergo chest x ray ,inflammed joints..look for progress of disease ..