CD8A and infection: However, despite this clear need for CD8+ T-cell–mediated immunity, infections due to a broad range of acute viruses, including influenza virus, respiratory syncytial virus (RSV), pneumonia virus of mice, vaccinia virus (respiratory but not systemic infection), human metapneumovirus (HMPV), and others, have been associated with a state called T-cell impairment [3–11].