<unk>  <unk>  was formally defined for the first time by Petersdorf and Beeson:<unk>  above 38.3° C (100.9° F) on<unk> al occasions, persisting without diagnosis for at least three weeks in spite of at least one week’s investigation in hospital.ʺ
Durack and Street made four different categories of<unk>  in 1991:
Fever of unknown origin
Rash with fever
Q fever
Relapsing fever
Pharyngoconjunctival fever
Morning sickness
Strep throat
Influenza-like symptoms