Kate1949 ditto. During the Asian flu epidemic of the late 50s, my sister and I were at boarding school, sleeping in open dormitories. People were going down with the flu left, right and centre. DS and I were completely unaffected. In the end the school closed for two weeks and we returned to home on an army base, again every one else around us was catching it, but none of us did.
I read somewhere, probably New Scientist years ag, that half th population handle the flu virus differently, not immune but just a dfferent boldily process. I think this means we get it but do not know we have it because are asymtopic.
I never used to bother with the flu vaccination until I read. last year that the vaccine as well as combatting flu also strengthens your immuns system, and for that reason, and that reason only, I have had the flu jab for the last two years.