This online resource explores different pathogens, how they are transmitted and how the immune system combats them. Communicable or infectious diseases cause millions of deaths globally every year. These are diseases that are caused by pathogens. The human immune system has evolved to protect us from infection as far as possible. Understanding how pathogens cause disease, and how a healthy body protects us against pathogens, can help us reduce the impact of communicable diseases around the world. This resource explores the roles played by pathogens and the immune system in combatting them.