The spike protein downregulates ACE2[1], which may make an otherwise benign respiratory infection severe[2].
1. SARS1 spike protein itself, without any virus, causes ACE2 downregulation which leads to severe lung damage: https://twitter.com/__philipn__/status/1237588716236898304
2. When mice are engineered to be deficient in ACE2, the otherwise non-severe RSV causes severe disease: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4728398/
(This may be why COVID is bad. Check out my twitter above for a lot on this)