Viruses in the cold waters of the Arctic are strongly seasonal and are also detected in the Antarctic. This surprising discovery comes from a multi-year time-series study led by the GEOMAR Helmholtz Center for Ocean Research Kiel. The results offer new insights into the fragile balance of polar ecosystems—with implications for the role of viruses as indicators of change in the ocean, about which baseline knowledge is still lacking. The study is published in Nature Communications.