For mice harvested at day 10 post-infection, mice were treated with VIP from day 5 to day 10 whereas for day 14 post-infection group, three regimens of VIP treatment were examined: day 5–14, day 5–10 and day 10–14, to investigate if we could locate a therapeutic window during the time where endogenous VIP was low and VIP supplement may be beneficial for the mucosal surface, without detrimentally affecting the actions of the immune response to clear the infection. Here, VIP is linked to infection.