The virus infecting humans that is closest to SARS-CoV-2 is SARS-CoV, the agent that caused the SARS outbreak in 2002 and 2003. These two viruses are very similar in their genomes, in their way of entering cells, and in some of their clinical characteristics. Since 2003, we have learned many things from the virus that caused SARS. We have learned how the virus enters cells, how it replicates, and how it interacts with the immune system. We have learned some of the main factors that contribute to the worsening of the disease. Animal models have been established, and therapies have been developed and proposed. This acquired knowledge can accelerate the discovery of potential treatments for COVID-19.