ERVW-1 and infection: The bursa of Fabricius is poorly developed and its structure does not differentiate into cortex and medulla (this is not the case in post-hatch infection, in which the bursa development is relatively normal [199]) due to the blocking of the differentiation of B-cell progenitors, causing an arrest of the development of B-cells and the inhibition of humoral immunity; in addition, ALV-J gp85 (SU) mediates B-cell anergy by inhibiting BCR signal transduction [199].