Denoting the virulence of a virus capable of inducing lethal infection in embryonic hosts after a long incubation period and an inapparent infection in immature and adult hosts; the term is used in characterizing Newcastle disease virus, particularly strains used as vaccines administered in water or as sprays, i.e., mild or avirulent strains. [L. lentus, sluggish, inactive, + G. -gen, producing]