Theory predicts that temperature fluctuations should reduce performance near an organism's thermal optimum. A new study in PLOS Biology found fluctuations increased parasite transmission instead, highlighting questions about how climate change will impact infectious diseases. [Abstract copyright: Copyright: © 2023 Marta S. Shocket. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.]