Corresponding to the delayed but severe pneumonia observed in low-dose infection in C3−/− mice and the increased bacterial load, levels of IFN-γ, TNF-α, IL-6 and IL-10 were significantly elevated in infected animals (Fig. 7, with cytokine levels in a similar range as observed on day 4 or 9 with the high IFU infection (Fig. 4). This evidence concerns the gene IL6 and infection.