In the absence of flow regions of cell infection are localized (white points). But in the presence of outward radial flows, regions of infection are spread in the direction of flow (white comets). Culture wells are 35mm in diameter. Details are Zhu et al. 2007. How do spontaneous fluid flows arise in culture wells? Evaporation of the fluid in well is more efficient at the center of the well, so that fluid at the center of the well is cooler, has a higher density, and falls, driving outward radial flows that sweep across the bottom of the well; details are in Lindsay et al. 2016.