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The U.S. Centers for Disease Control and Prevention told us last weekend that America's AIDS epidemic is 40 percent larger than it had believed, and that black America accounts for about 45 percent of new infections each year. If we're only 13 percent of the population, how can that be?

There's no one simple answer. It takes a witch's brew of toxins to create a disparity that large.

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The first thing to understand is that it's not new. Blacks made up a quarter of all Americans who died from AIDS as early as 1985, though we were less than 12 percent of the population at the time. Eleven years later, in 1996, the American epidemic hit two milestones: It was both the first year in which fewer people died than the year before, and the first year in which more blacks died than whites. Since then, the disproportionate impact on blacks has grown steadily, where we now make up 40 percent of AIDS deaths annually.

These out-of-whack death rates offer a window into infection rates, too. Studies reveal that many blacks don't find out they're infected until it's too late, after they've gotten sick and treatment is more difficult. Why does that matter to the virus' spread? Because it also means they've been walking around with the virus for years, unaware of their risk to others.

The CDC says at least a quarter of all HIV-positive Americans don't know they're infected—and roughly half of those people are black. These folks are driving new infections, researchers believe. Successful treatment significantly lowers the amount of HIV in a person's blood, thus making that person much less infectious.

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Similarly, African Americans, particularly young black women, have higher rates of undiagnosed STDs than their peers. And that increases the risk of infection. While it's true that you can contract HIV from a single exposure, the odds are against it when we're talking about sex. Absent a direct route to the blood stream—such as injecting drugs with a used needle—the rate of infection upon exposure is low, relative to other sexual infections. But add an untreated STD to the mix and the likelihood you'll contract HIV, as well, goes up at least five-fold.

So the severity of the black AIDS epidemic is fueled by the same factors that drive a host of other health problems have hit us harder: We either don't have access to consistent, preventive health care, or we don't use the health care we've got.

Which brings us to the broader social and economic issues driving the racial disparity in HIV infection. Yes, access to and attention to health care are central issues. But there's so much more.

One fascinating theory gaining credibility and drawing research attention focuses on what public health experts call "sexual networks"—or, the universe of people from which someone is likely to choose a sex partner. The dynamics of a given sexual network—and we've all got one—will affect the level of risk for HIV (and other STDs) that individuals in that network face.

And, to put a very fine point on it, there is much about the range and dynamism of black sexual networks that contribute to HIVs spread.

The biggest factor is that black sexual networks appear to be smaller and more segregated than other racial groups. That means that once HIV is introduced into the circle, it can spread more quickly.

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The fact that black sexual networks are smaller and more segregated also makes them more egalitarian. When targeting resources, public health officials seek to identify what they call "core" members of a network. These are the ones who are taking the most sexual risks, such as juggling multiple partners at one time. In other communities, those folks tend to have sex only with each other, and thus the risk remains contained. Not so in black communities, where there is far greater mixing in all aspects of life, including sex. That helps explain why, despite the far higher infection rate, blacks overall report no more risky sexual behavior than other groups.

But the cutting edge sexual-network researchers are considering how public policy, particularly criminal justice, turned what should be idle trivia about mating habits into life or death information. At the current rate of incarceration, one in three black men will spend time locked up before they die. And it looks like moving black bodies in and out of neighborhoods with that sort of rapidity has made the whole community unhealthy.

There's the question of HIV transmission inside prisons. What we know is that HIV prevalence—the percentage of people infected—is five times higher behind bars than in the overall population, according to the CDC. Prison officials argue that it's because the people getting locked up are more likely to have HIV in the first place, and at least one CDC study supports that assertion. Others, however, note that condoms are banned inside all but two state prison systems and clean needles are banned in all of them. With sex and drugs readily available and HIV widespread, it's hard to imagine people aren't contracting HIV.

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Set that debate to the side, though, and the high incarceration rate of black men still appears to impact the overall black HIV infection rate. By moving so many men in and out of already small, segregated sexual networks, the criminal justice system drives multiple, overlapping relationships. A boyfriend gets locked up for a couple of years, and his girlfriend gets a new boyfriend. He gets out and they get back together, but in the meantime she's had two sexual partners instead of one. Some researchers argue this sort of serial monogamy inside a small group fuels HIV's spread.

A common thread runs throughout these seemingly diverse theories: HIV attacks black America as a communal whole. And that fact points to what may be the most honest answer to the question of why blacks are so much more affected. In black America, HIV is a disease that binds us all together, but we've spent most of the last three decades distancing ourselves from it as someone else's problem.

Kai Wright is author of Drifting Toward Love: Black, Brown, Gay and Coming of Age on the Streets of New York .