Among women with CD4 counts <350, the cumulative probability of postnatal HIV transmission by 4 months was 0.191 (95% CI: 0.095–0.288) for non-EBF (n = 65) and 0.063 (95% CI: 0.036–0.091) for EBF (n = 306); RH = 3.40 (95% CI: 1.58–7.33) after adjusting for viral load, syphilis status and low birthweight. The gene discussed is CD4; the disease is syphilis.