Whether it's working to ward off the common cold or battling a serious condition, the immune system is a powerful network that sustains and protects us during all stages of our lives. This special edition of Health delves into this system, including science-backed answers to questions about the immune system and how it works, the secret life of germs, invisible illnesses, a flu vaccine primer, and our list of the best immune-boosting foods. Also included is a primer on Covid-19; what we’ve learned and what’s on the horizon.