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“The only thing necessary for these diseases to the triumph is for good people and governments to do nothing.”


Identifying high-risk carriers of infectious diseases is worth the effort.

20 August 2001

              PHILIP BALL


Sexually transmitted diseases (STDs) call for discrimination. Containing the spread of an STD by  focusing on promiscuous individuals, who are most  likely to pass it on, should be cheaper and more effective than large-scale random campaigns, according to two new mathematical analyses1,2.Why? Because the web of  human sexual contacts is scale-free - there is no typical number of sexual partners3. Many people have

              few partners; a few have many. And diseases propagate

              differently through scale-free networks than through networks

              in which contacts between individuals are purely random.


              An epidemic spreads through a random network only when

              the disease is transmitted faster than a certain threshold

              value. A disease can be eliminated from a randomly

              connected population by keeping the transmission rate below

              this threshold, for example by immunization.


              But there is no such threshold in scale-free networks, so

              even a very slow-spreading disease can be sustained at a

              low incidence throughout the population4. And calculations

              now show that uniform, random immunization would fail to

              eradicate the disease.



              The upside to scale-free networks is that they are

              characterized by a scattering of very highly connected nodes

              - 'hubs' that hold the web together. The hubs in this case are

              individuals who have many sexual contacts.


              So immunizing promiscuous individuals could effectively

              curtail transmission of an STD at relatively little cost. In other

              words, by severing the hubs' connections, the web rapidly

              falls apart, say Romualdo Pastor-Satorras of the Universitat

              Politècnica de Catalunya in Barcelona, Spain, and Alessandro

              Vespignani of the Abdus Salam International Centre for

              Theoretical Physics in Trieste, Italy.


              The problem is finding the hubs - promiscuous individuals

              are notoriously hard to identify. Fortunately, as Zoltán Dezsö

              and Albert-Lá¡szló Barabási of the University of Notre Dame

              in Indiana show, any targeting of hubs, however imperfect,

              raises the threshold spreading rate above zero, offering a

              chance to stamp out the disease for good.



              "Even modestly effective attempts to uncover and treat the

              hubs, if carried out systematically, are more successful than

              policies based on large-scale but random distribution of the

              available treatments," say Dezsö and Barabási. If strongly

              focused, control and prevention campaigns should work even

              if they don't always hit their targets, both teams of

              researchers agree.






                 1.Pastor-Satorras, R. & Vespignani, A. Optimal immunisation

                   of complex networks. Preprint, July, (2001).

                 2.Dezsö, Z. & Barabási, A.-L. Can we stop the AIDS epidemic?

                   Preprint, July (2001).

                 3.Liljeros, F., Edling, C. R. , Amaral L. A. N., Stanley H. E. &

                   Aberg Y. The web of human sexual contacts. Nature, 411,

                   907 - 908 (2001).

                 4.Pastor-Satorras, R. & Vespignani, A. Epidemic spreading in

                   scale-free networks. Physical Review Letters, 86, 3200 -

                   3203 (2001).


              © Nature News Service / Macmillan Magazines Ltd 2001