In patients with Covid-19, the immune system's T cells learn to recognize and target the new coronavirus. But some people who were never infected with the virus nonetheless have T cells that also recognize it. Researchers had suspected that in these individuals, past exposure to other coronaviruses, such as those that cause the common cold, had somehow primed their T cells to recognize and attack this new coronavirus (SARS-CoV-2), and new research appears to confirm that. In studies of human blood samples collected well before the new coronavirus was discovered, researchers found T cells that were equally reactive against the new virus and four types of common cold coronaviruses. The strongest T cell responses to the new coronavirus were associated with the spike protein the virus uses to enter human cells.