5. The absence of infectious agent in EMB is defined by negative results of PCR testing of EMB specimens. PCR testing will be aimed to exclude the presence of enteroviruses (ECHO, coxsackie), adenoviruses, herpes viruses (herpes simplex virus (HSV-1), Epstein-Barr virus (EBV), cytomegalovirus (CMV), human herpes virus (HHV-6)), Borrelia burgdorferi and parvovirus B19. In the case of parvovirus B19, a negative PCR result will be considered when less than 500 viral copies/ug genomic DNA are detected. EMB must have been performed no more than 6 weeks prior to the inclusion in the study.