M icrobes transmitted to mammals by arthropods contend with many factors that could impede survival. To survive, host fitness with infection must outweigh costs. In this issue of the JCI, Neelakanta et al. demonstrate that ticks infected with Anaplasma phagocytophilum show enhanced fitness against freezing injury owing to induced expression of tick “antifreeze glycoprotein.” This allows A. phagocytophilum to successfully propagate and survive to cause disease in nonnatural hosts, such as humans. How an intracellular microbe with a small genome subverts host cell function for survival provides insight into the control of some cellular function programs and underscores how vector biology can have an impact on human health.