A study published in Nature reports that common respiratory infections, including COVID-19 and influenza, can reactivate dormant breast cancer cells in the lungs and prompt rapid tumor growth. After primary treatment, inactive disseminated cancer cells can remain quiet for years without producing symptoms. Severe inflammation triggered by respiratory viruses, however, can stimulate these dormant cells to resume division and form aggressive metastatic tumors within weeks. Investigators identified interleukin-6 (IL-6), an inflammatory protein released during viral infection, as the main molecular signal driving this reactivation. Laboratory results in mouse models align with observational human data. Breast cancer survivors who contracted COVID-19 showed a higher risk of developing lung metastases than those who did not become infected, with some analyses indicating an increase of roughly 40 to 50 percent. The researchers note that the findings, while concerning, also point toward possible protective measures. Blocking IL-6, providing timely care after infection, or maintaining routine vaccinations against respiratory viruses could reduce the inflammatory stimulus and lower the chance of recurrence in cancer survivors. [Chia, S. B., Johnson, B. J., Hu, J., et al. (2025). Respiratory viral infections awaken metastatic breast cancer cells in lungs. Nature, 645, 496–506. DOI: 10.1038/s41586-025-09332-0]