However, only mild diarrhea occurred in 2-and 4-week-old pigs and no clinical signs were observed in 8-and 12week old pigs (Shibata et al., 2000) . In our studies, morbidity was consistently 100% in the PC21A or PC22A-inoculated piglets and their contact-exposed sows (Lin et al., 2015a; Liu et al., 2015) . In our laboratories, a slower turnover rate of enterocytes and impaired lytic activity and IFN-gamma production by natural killer cells in 9-day-old nursing piglets being less than in 26-day-old were proposed as explanations for one of the mechanisms of age-dependent resistance to PEDV.