A weak immune system is like a passive invitation for germs to invade our body. The body's sole defence mechanism, the immune system, always remains alert against foreign attack. It memorises the pathogen and protects the body from future attacks. Usually this memory is naturally induced after the body is exposed to the pathogen or else the immune system's memory is artificially induced with the help of vaccines.

from Times of India https://timesofindia.indiatimes.com/life-style/health-fitness/health-news/covid-infection-signs-you-have-a-weak-immune-system/photostory/90640606.cms