A number of bacterial and viral pathogens are able to divert the actin-based motility machinery of infected cells to generate a propulsion system that facilitates microbial dissemination. In those instances, pathogen-induced nucleation of actin filaments gives rise to 'comet tails' that appear to trail the invader as it moves toward the cellular periphery. Although the cellular components involved in comet tail generation are known, exactly how actin filaments organize themselves to generate directional movement remains unclear. Small and colleagues now expose the mechanics of actin-based pathogen propulsion with a combination of cryo-electron tomography and mathematical modeling. Baculoviruses normally infect insect cells, but they can also generate actin comet tails in human cells, where the filaments can more easily be analyzed. Using this system, the authors were able to obtain detailed cryo-tomographic views of virus-induced comet tails. The structures revealed fishbone-like arrangements of short actin filaments generated by frequent branching, with the fast polymerizing plus end of the filaments directed forward. The authors also carried out simulations based on stochastic mathematical models. When certain assumptions were used, the simulations could reproduce both the experimentally observed actin comet structures and the typical path taken by the virus while being propelled through the cytoplasm. A model in which the actin filaments proximal to the virus were continuously attached to, and branching was biased toward, the viral surface best reproduced the experimental observations. This mechanism of propulsion, in which a few proximal filaments push the virus through their fast polymerizing ends, is likely to apply to other pathogens such as the bacterium Listeria monocytogenes. (PLoS Biol. doi:10.1371/journal.pbio.1001765, 14 January 2014)