Why do some people have asymptomatic Covid-19, while others suffer from severe or life-threatening disease? Many risk factors for severe Covid have been identified – age, obesity, sex, smoking status, etc. – but this does not fully account for the observed heterogeneity. One possibility is that lifetime exposure to other coronaviruses – which can cause the common cold – may confer partial immunity to SARS-CoV-2 (the virus that causes Covid-19), resulting in asymptomatic infection. There is evidence both for and against this idea. The first link highlights recent evidence from U Penn that prior exposure to other coronaviruses does not protect you from severe Covid-19, but these questions are difficult to answer with clarity and we still have much to learn. U Penn
However we can extend this idea to another level: what if your recent ancestors were exposed to coronaviruses similar to SARS-CoV-2, and this provided you – in your lifetime – with partial immunity to SARS-CoV-2 via trans-generational inheritance? Recent research (Pasteur Institute) from fruit flies and mosquitoes demonstrates that this is indeed a mechanism of immunity used by Mother Nature to combat single-stranded RNA viruses similar to SARS-CoV-2. In addition, they demonstrate that the immunity can last for several generations.