This lecture explores the dynamic interactions between hosts and parasites, focusing on the diversity of host-pathogen interactions and the trade-off between investing in the immune system and other physiological functions. It discusses the various pathogen strategies and the immune response adapted to combat them, including resilience, avoidance, and immunity. The risks faced by immunocompromised individuals, the importance of protective immunity through vaccines, and the role of the immune system in defending against infections and tumors are also covered. Additionally, it delves into immune deficiencies, autoimmune diseases, and allergies as part of the immune system's physiological function.