Treatment coverage (i.e., the percentage of discordant couples that receive antiretroviral therapy) ranges from 0 to 100%. Results are shown for Lesotho (black), Malawi (red), Rwanda (blue), and Ghana (green). The effect of treatment coverage on the annual incidence rate (per 1000 person-years) (a) and (b) and on the annual number of infections prevented (c) and (d). Calculations shown in (a) and (c) are based on the assumption that 40% of the population is in stable partnerships; stable partnerships include concordant positive, concordant negative and discordant couples. Calculations shown in (b) and (d) are based on the assumption that 70% of the population is in stable partnerships.