Infectious bursal disease (IBD) virus (IBDV, genus Avibirnavirus, family Birnaviridae) infects chickens, turkeys, ducks, guinea fowl and ostriches, but causes clinical disease solely in young chickens. Severe acute disease, usually in 3- to 6-week-old birds, is associated with high mortality, but less acute or subclinical infections are common earlier in life. IBDV causes lymphoid depletion in the bursa of Fabricius. Significant depression of the humoral antibody responses may result, thus promoting secondary infections. Two serotypes of IBDV, designated serotypes 1 and 2, are recognised. Clinical disease has been associated only with serotype 1, against which all commercial vaccines are prepared. Some antigenic variants of serotype 1 IBDV may require special vaccines for maximum protection. Very virulent strains of serotype 1 IBDV are common worldwide and cause serious disease. IBD has not been reported to have any zoonotic potential.