Ascertaining the burden of invasive Salmonella disease in hospitalised febrile children aged under four years in Blantyre, Malawi by Msefula, CL, Olgemoeller, F, Jambo, N, Segula, D, Van Tan, T, Nyirenda, TS, Nedi, W, Kennedy, N, Graham, M, Henrion, MYR, Baker, S, Feasey, N, Gordon, M, Heyderman, RS