The immune system is among the most intricate of all the body's systems. It is an elaborate network of specialised cells, originating in the bone marrow and thymus, that carries defensive signals (1)---- the entire body. It relies on a host of chemical messengers to coordinate a response to threats that we broadly (2)---- as infection, injury, or disease. In fact, at this very moment, (3)---- a remarkable coordination between white blood cells and the nervous system, our bodies are constantly monitoring for potential harm. Vaccines (4)---- on the same fundamental principle as natural immunity. (5)----, they have never been able to fully replace the protection offered by an infection that has been survived naturally.