MUC5AC and viral infectious disease: This was defined as an ‘intermediate' cell type with characteristics of both goblet cells and club cells.54 Our results suggest that a proportion of club cells may undergo differentiation toward a goblet cell morphology, and may be in part the mechanism of mucous metaplasia observed in response to viral infection.55 The increase in BPIFA1 expression seen here at day 14 p.i. may also be associated with the phenotypic alteration of club cells into mucin-secreting cells.