However, frequency of CD4+ ICOS+ T cells decreased after 7th day post infection (Additional file 1: Figs. S1, S2), in animals infected with non-lethal malaria parasite as compared to lethal parasite with gradual increase in the number of these cells as shown in Fig. 2A, B. These observations indicate that lethal and non-lethal malaria parasite differentially regulates the expression of co-stimulatory molecules (ICOS) on CD4+ T cells. Here, ICOS is linked to infection.