Cells regularly experience fluid flow in natural systems. However, most experimental systems rely on batch cell culture and fail to consider the effect of flow-driven dynamics on cell physiology. Using microfluidics and single-cell imaging, we discover that the interplay of physical shear rate (a measure of fluid flow) and chemical stress trigger a transcriptional response in the human pathogen . In batch cell culture, cells protect themselves by quickly scavenging the ubiquitous chemical stressor hydrogen peroxide (H O ) from the media. In microfluidic conditions, we observe that cell scavenging generates spatial gradients of H O . High shear rates replenish H O , abolish gradients, and generate a stress response. Combining mathematical simulations and biophysical experiments, we find that flow triggers an effect like "wind-chill" that sensitizes cells to H O concentrations 100 to 1,000 times lower than traditionally studied in batch cell culture. Surprisingly, the shear rate and H O concentration required to generate a transcriptional response closely match their respective values in the human bloodstream. Thus, our results explain a long-standing discrepancy between H O levels in experimental and host environments. Finally, we demonstrate that the shear rate and H O concentration found in the human bloodstream trigger gene expression in the blood-relevant human pathogen , suggesting that flow sensitizes bacteria to chemical stress in natural environments.