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Illustrated are three different strategies for preventing the spread of a disease across a simple social network by targeting vaccine shots to a limited number of people (nodes marked red). Picking people at random (top) doesn’t help the others — the disease can still spread freely. Vaccinating the people who are most connected (center) leaves a large network where the disease could still spread, while some nodes are, unnecessarily, completely isolated. A targeted approach that partitions the network into roughly equal parts (bottom) gives a better chance at protecting the most people. Credit: Yiping Chen, Boston University