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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