An algorithmic search for community bridges as described above can potentially take a very long time, depending on the structural features of the network. For example, the frequency of nodes that can potentially meet the immunization requirement set by the algorithm might be smaller than the desired immunization coverage. To prevent endless searches for community bridges, two additional checks are implemented. First, the number of nodes in any running random path does not exceed 10 (this is implemented using a first-in-first-out list that keeps track of the visited nodes). Second, we keep track of all nodes visited, and if a node has been visited at least k times (on any random walk), it will be immunized. In all results presented in this manuscript, we use k = 2. Figure S1 Results from simulations with the same parameters and settings as Figure 1a in the main text, but based on networks with lower community structure. The initial creation of these networks was identical to those created for Figure 1 in the main text (see description in Methods in the main text), but rather than rewiring between-community edges and turn them into withincommunity edges, we randomly rewired within-community edges in the following way: at each rewiring step, we (i) randomly choose a within-community edge, (ii) randomly choose one of the two nodes, (iii) pick a random node in the network, and rewire the edge by detaching it from the node that was not chosen in step (ii), and attaching it to the new node that was chosen in step (iii). At all times, edges must always fall between two distinct nodes, and there can only be one edge between any two pair of nodes. Note that this algorithm is essentially the reverse of the algorithm used to create networks with increased community structure in the main text.  Generation of Human CEACAM1 Transgenic Mice and Binding of Neisseria Opa Protein to Their Neutrophils BACKGROUND: Human CEACAM1 is a cell-cell adhesion molecule with multiple functions including insulin clearance in the liver, vasculogenesis in endothelial cells, lumen formation in the mammary gland, and binding of certain human pathogens. PRINCIPAL FINDINGS: Three genomic BAC clones containing the human CEACAM1 gene were microinjected into pronuclei of fertilized FVB mouse oocytes. The embryos were implanted in the oviducts of pseudopregnant females and allowed to develop to term. DNA from newborn mice was evaluated by PCR for the presence of the human CEACAM1 gene. Feces of the PCR positive offspring screened for expression of human CEACAM1. Using this assay, one out of five PCR positive lines was positive for human CEACAM1 expression and showed stable transmission to the F1 generation with the expected transmission frequency (0.5) for heterozygotes. Liver, lung, intestine, kidney, mammary gland, and prostate were strongly positive for the dual expression of both murine and human CEACAM1 and mimic that seen in human tissue. Peripheral blood and bone marrow granulocytes stained strongly for human CEACAM1 and bound Neisseria Opa proteins similar to that in human neutrophils. CONCLUSION: These transgenic animals may serve as a model for the binding of human pathogens to human CEACAM1. Introduction CEACAM1 (carcinoembryonic antigen related cell adhesion molecule-1) is a type 1 transmembrane protein expressed on most epithelial cells, phagocytes and activated lymphocytes [1, 2] . In the case of epithelial cells, its cell adhesion activity is associated with diverse functions including angiogenesis [3, 4] , lumen formation [5, 6, 7] and the binding of extracellular pathogens [8] . In the case of neutrophils, it may play a role in the binding of neutrophils to activated endothelium and extravasation into inflamed tissues [9, 10] , and similar to epithelial cells, may bind extracellular pathogens [8, 11, 12] . In the case of activated lymphocytes, it serves as an inhibitory molecule, controlling the strength and duration of the immune response [2] . CEACAM1 is a critical molecule well situated to play a communication role between epithelial cells, that represent the first barrier to infection, and immune cells that control infections that disrupt the epithelial barrier. Thus, pathogens that bind to CEACAM1 may interfere with its normal function, namely maintenance of communication between these two barriers to infection. CEACAM1 has multiple splice forms that are differentially expressed between epithelial and immune cells. The so-called ''long form'' has a cytoplasmic domain containing two ITIMs (immunoreceptor tyrosine-based activation motifs) that, when phosphorylated, may bind Src homology 2 domain-containing tyrosine phosphatases SHP1 and SHP2 [13] to convey inhibitory signals to the T-Cell receptor [14, 15, 16] , B-Cell receptor [17] , and epithelial growth factor receptor [18] . The long form also has a bcatenin binding site [19] and a filamin A binding site [20] . The socalled ''short form'' lacks the ITIMs, but has the ability, like the long form, to bind actin, calmodulin, and annexin II [21] . These activities indicate a strong association of CEACAM1 with actin cytoskeleton [7, 21, 22] .