United Press International - Monday, 23 July 2001
Mike Martin

Until now, the best arguments for providing costly AIDS drugs to impoverished African and Asian nations drowning in the illness were humanitarian.

Albert-Laszlo Barabasi and Zoltan Deszo have now provided what may be the first convincing scientific evidence that slowing acquired immune deficiency syndrome in developing nations may actually halt its global course. They said poorer nations represent highly concentrated "hubs" or disease centers with just enough global connectedness to make eradicating the disease within their borders absolutely essential.

"The continued spreading of the HIV virus is remarkable because relatively effective therapies are available that not only expand the lifetime of infected individuals, but also lower the transmission probability," Barabasi said in a recent paper on the subject. "The problem is that these expensive therapies are beyond reach in developing countries."

Building on previous work by Boston University researchers Luis Amaral, H.E. Stanley and coworkers Barabasi and Deszo claim newly quantified patterns of human sexual contact create a "node-hub" architecture more typical of a computer network than a populated community.

These hubs and nodes, they said, are so concentrated and yet so connected that eradicating any sexually transmitted disease requires concentrated efforts at the hubs. As in any computer network -- or a chain of Christmas lights --interrupting the flow of information or electricity at a hub can cause flows to cease network wide.

So too, Barabasi and Deszo claim, with AIDS. Slow or stop the disease at a hub, they said, and you severely reduce its ability to spread anywhere else.

"We are indeed familiar with the results by Barabasi and co-workers and are certainly excited by all the developments coming out," Luis Amaral told United Press International. "We believe that the new focus on the possibility of characterizing the structure of networks could help the understanding of epidemics both for human and animal diseases."

The discovery of this new pattern of epidemic transmission is a major step in a worldwide effort. Amaral and Stanley revealed humans engage in "scale-free" patterns of intimacy: A very few individuals have the largest number of sexual contacts.

Sociologists and epidemiologists previously had taken for granted the idea that human sexual activity followed a standard bell-shaped curve: The largest clusters of people would have an average number of sexual contacts with very few people engaging in either very few or very many sexual encounters at either ends of the curve.

Instead, the Boston researchers found a curve that gradually curves upward and keeps rising. Most surprising, a very few 10 percent of men have 48 percent of all sexual encounters, a pattern more like the distribution of wealth where 1 percent of people control 95 percent of all assets.

Capitalizing on this discovery, Spanish scientists Yamir Moreno, Romualdo Pastor-Satorras and Alessandro Vespignani showed that "scale-free" transmission patterns allow even the weakest infective diseases to spread unchecked. The frightening upshot is that given the scale-free pattern of human sexual contact, "short of a cure or vaccine available to all, the HIV virus will eventually reach the so far uninfected segments of the population exposed to the disease," Barabasi explained in a recent paper on the topic.

Now Barabasi imposes a network architecture on AIDS transmission patterns. "Epidemics spread without a threshold on a scale-free network thanks to hubs and nodes with an unexpectedly large number of links," Barabasi said. "Once infected, hubs offer an efficient conduit for disease spreading by reaching an unusually high percentage of other nodes."

As a result, anything less than attacking the disease at its hub represents random treatments that cannot contain the epidemic because, Barabasi said, they "leave the scale-free nature of the network unaltered."

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