A virus lacking K7 replicated normally in cell culture (Fig. 3) and in vivo (Fig. 5c d 2), but was less virulent than control viruses in both i.d. (Fig. 4) and i.n. (Fig. 5a, b) infection models and induced stronger pulmonary NK and CD8+ T-cell responses (Fig. 7). Here, CD8A is linked to infection.