A 52 year old patient with immunosuppressive therapy for ANCA-negative vasculitis presented with fever and paraparesis of the legs, laboratory findings displayed high inflammatory markers. The differential diagnosis comprised acute infection, an exacerbation of vasculitis or a drug reaction. Despite meticulous diagnostics, including FDG-PET, no definitive cause for the symptoms could be uncovered. Empirical antibiotic treatment in combination with high doses of glucocorticosteroids lead to prompt resolution of fever and inflammatory laboratory markers.