One in five

HIV may be harder to measure than anything else we study in public health and social science. It is largely invisible to the naked eye, its transmission is low-probability and highly variable, and perhaps most importantly, people have strong incentives to hide their status and avoid testing. These incentives arise from everything from stigma to a desire to maintain access to sex partners to a perception that HIV testing is just a death sentence, with tests highly likely to be positive and bringing no benefits. We’ve gotten a lot better at measuring things like the prevalence of the virus, but even now our estimates have to be taken with a grain of salt.

Despite all that, we do have a decent sense of how common it is in different regions of the world. In urban Malawi, which includes the place I’m currently sitting, the answer is that a little over 20% of the population has the virus. For historical reasons, those rates are typically computed for the population aged 15-49, so the overall rate might differ, but that’s still a decent range for prime-aged adults (and indeed there’s no reason to think the prevalence drops drastically above that age range). HIV takes around ten years to lead to death, with visible AIDS lasting for something like one year. So as a basic estimate, 20% of people are carrying the virus, but just 2% would be visibly ill. That second number is small enough to ignore, so about one out of every five people in Lilongwe is infected with HIV.

This is a nearly-incomprehensible figure for me. Sometimes if I pass through crowds I try to imagine that every fifth person I pass has an incurable virus (although many don’t know it). I honestly can’t do it – they all look like regular people to me, because that is exactly what they are. But all the evidence I’ve seen indicates that people here do manage to wrap their minds around the idea that one in every five people in the street (or one in every five potential sex partners, or one in every five people on their soccer team) is infected with HIV. I have no idea how they manage it, either psychologically or in terms of making important life decisions about stuff like whom to marry. People often wonder what an economist is doing studying HIV prevention. The answer is that the choices people make about sex and marriage in the face of HIV constitute what I would argue is the hardest – and the most interesting – decisionmaking problem in the world.

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