Food security reduces multiple HIV infection risks for high-vulnerability adolescent mothers and non-mothers in South Africa: a cross-sectional study by Cluver, L, Rudgard, WE, Toska, E, Orkin, M, Ibrahim, M, Langwenya, N, Kuo, C, Xaba, N, Roehm, K, Smith, M, Bernardini, S, Giordana, G, Mumma, M, Kingori, J, Yates, R, Sherr, L