One of the fascinating features of the biology of endogenous avian leukosis virus (ALV) is that these viruses, when introduced into chickens as exogenous infections, do not cause disease (Motta et al. 1975; Purchase et al. 1977; Crittenden et al. 1979a,b). This is in striking contrast to ALVs of exogenous origin, which are associated with a wide spectrum of acute and nonacute diseases (Purchase and Burmester 1977; Duesberg, this volume).
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Robinson et al. (1980) studied this question.