If it's something you have encountered before and become infected by, you immune system has provided you with antibodies and can deal with any exposure. This doesn't always have to be the identical virus (see smallpox and cowpox), so there is a possibility that previous infection with a strain of the common cold or SARS, may provide some immunity to COVID19
At the end of February, my husband and our two sons went to Malta for a week - so spent a week abroad and also 4 trips to an airport.
Anyhow - a week after they came back I got a phone call from my son’s (year 1) school asking me to pick him up because he had a temperature and didn’t feel well. When I collected him he was crying, said his throat hurt too much to talk and had a temperature of 38.8. He was unwell with these symptoms for about 3 days. At about the same time my husband came down with the most awful cough which lasted for about 3 weeks. About 5 days after my first son came down with his symptoms my youngest son, aged 2, also started getting temperatures, though not as high as my other son’s was.
When news of the Corona virus came out I was joking that they all must have had it but concluded it was just a coincidence as I hadn’t been ill whereas if they’d have spread it to me then I would have.
However, I’m a nurse, and over the last 10 years I have spent every winter exposed to God knows how many respiratory viruses, included lots of exposure to the corona virus.
Do you think it could be possible then that due to my job history I may have some degree of immunity to Covid-19?