The recent pandemic has revealed the fragile balance between humans and the microbes that surround us. Key to setting this balance is the immune system and the ability of that system to lay down memories. Although such immunity is typically highly effective, pathogens have also evolved with us to explore and exploit loopholes, allowing them to escape host control. In this talk we will look at the basic building blocks of effective immune responses, how microbes, especially viruses, can set up persistent infections in individuals or across communities – and what we can do about it.