The defences of airways against enveloped RNA viruses include the activation of the HO-1/CO/H2S system and the release of H2S. A bulk of available evidences support the idea that the infection from SARS-CoV-2 responds to these mechanisms and the epidemiology, clinical manifestations, co-morbidities and risk factors for Covid 19 could be explained by a failure of this system. Here, HMOX1 is linked to infection.