Infection assays enable quantification of virus growth, testing of anti-viral drugs and management of viral diseases, but established assays are labor-intensive to perform. Here we exploit microscale fluid flows to promote the spread of viruses from single infected cells across susceptible cell monolayers, creating macroscopic comet-shaped plaques (or ‘comets'). Vesicular stomatitis virus (VSV) infects baby hamster kidney (BHK-21) cells in standard six-well culture plates, forming comets within 12 hours post-infection. Outward radial flows, measured by tracking fluorescent beads, are 140 microns per hour near the wall, and consistent with a mechanism of evaporation-induced flow. Inhibition of comet formation by an antiviral drug (5-fluorouracil), quantified using a digital scanner, produced a measure of drug susceptibility that was 4- to 16-fold more sensitive than standard yield- and plaque- reduction assays, respectively, while eliminating the need for plaque counting. This quantitative comet assay may be adapted to influenza virus, human cytomegalovirus and other plaque-forming viruses. Further, microfluidic techniques may enable us to localize infection in microchannels as well as control flows carrying the viral particles, and thus develop a high-throughput assay to test viral drug susceptibility.