Hi My Husband Age 37 Recently Had Fever One Month

Hi my husband age 37 recently had fever one month back and was lasted for abt 15 days first two days o f fever around 101 -100 and then 100 for other 3 days nd from then 99 for abt 7 days usually evening in last 7 days.all tests r normal widal,dengue,cat scan brain and cbp and cue.but again after 15 days again today he got fever abt 99.2 with leg pains.wht could be the reason