high-risk carriers of infectious diseases is worth the effort.
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
in which contacts between individuals are purely
An epidemic spreads through a random network only when
the disease is transmitted faster than a certain
value. A disease can be eliminated from a randomly
connected population by keeping the transmission rate
this threshold, for example by immunization.
there is no such threshold in scale-free networks, so
even a very slow-spreading disease can be sustained at
low incidence throughout the population4. And
now show that uniform, random immunization would fail
eradicate the disease.
The upside to scale-free networks is that they are
characterized by a scattering of very highly connected
- 'hubs' that hold the web together. The hubs in this
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
falls apart, say Romualdo Pastor-Satorras of the
Politècnica de Catalunya in Barcelona, Spain, and
Vespignani of the Abdus Salam International Centre for
Theoretical Physics in Trieste, Italy.
The problem is finding the hubs - promiscuous
are notoriously hard to identify. Fortunately, as Zoltán
and Albert-Lá¡szló Barabási of the University of
in Indiana show, any targeting of hubs, however
raises the threshold spreading rate above zero,
chance to stamp out the disease for good.
"Even modestly effective attempts to uncover and
hubs, if carried out systematically, are more
policies based on large-scale but random distribution
available treatments," say Dezsö and Barabási.
focused, control and prevention campaigns should work
if they don't always hit their targets, both teams of
1.Pastor-Satorras, R. & Vespignani, A. Optimal
of complex networks. Preprint, July, (2001).
2.Dezsö, Z. & Barabási, A.-L. Can we stop the
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
scale-free networks. Physical Review Letters, 86, 3200
© Nature News Service / Macmillan Magazines Ltd 2001