Networks portray a multitude of interactions through which people meet, ideas
e spread, and infectious diseases propagate within a society. Identifying the
st efficient "spreaders" in a network is an important step to optimize the
e of available resources and ensure the more efficient spread of information.
re we show that, in contrast to common belief, the most influential spreaders
a social network do not correspond to the best connected people or to the
st central people (high betweenness centrality). Instead, we find: (i) The
st efficient spreaders are those located within the core of the network as
entified by the k-shell decomposition analysis. (ii) When multiple spreaders
e considered simultaneously, the distance between them becomes the crucial
rameter that determines the extend of the spreading. Furthermore, we find
at-- in the case of infections that do not confer immunity on recovered
dividuals-- the infection persists in the high k-shell layers of the network
der conditions where hubs may not be able to preserve the infection. Our
alysis provides a plausible route for an optimal design of efficient
ssemination strategies.