Label-free biodetection schemes compatible with standard CMOS fabrication methods constitute an important goal, as these are enabling tools for the mass production of high-sensitivity biosensors. Two-dimensional slab photonic crystal (2D slab-PhC) sensors have been posited as ultrahigh-sensitivity detection components, but to date recognition-mediated detection of viruses or simulants under flow has not been demonstrated. We report the design and optimization of a new W1 waveguide-coupled 2D slab-PhC sensor, with a geometry well suited to virus detection. Proof of concept experiments with fluorescent latex particles verified that the sensor could respond to infiltration of a single particle, both in air and under an aqueous cover layer. Subsequent experiments with antibody-functionalized sensors and virus simulants confirmed the ability of the device to detect virus-sized particles under flow via a recognition-mediated process. This work sets the stage for incorporation of 2D slab-PhC sensors into fully integrated photonic sensor systems.