The appropriate balance of HIV prevention and care activities depends on the specific epidemiology of HIV, including the stage of the epidemic; information about the cost-effectiveness of specific prevention interventions and treatment regimens; and the level of public resources available. In all countries, it is far less costly to prevent HIV than to treat people with AIDS. Once prevention needs are met, governments facing a severe epidemic who have additional resources should consider cost-effective treatment for people living with AIDS. The epidemiology of HIV provides two clear messages for prevention. First, it is imperative to act early in the epidemic, when HIV spreads exponentially. Viral load is highest during the first few months of infection, so that early in the epidemic a large proportion of those infected may be highly infectious. Second, it is crucial to target interventions initially to those with the highest-risk behavior. This will have the greatest impact on the number of new HIV infections prevented, as individuals with large numbers of sexual and needle-sharing partners who do not use condoms or clean injecting equipment are those most likely to become infected and then spread HIV. Changing the behavior of these individuals, even if only a relatively few members of society, is essential to curbing the epidemic.