Worldwide, infectious diseases are the second leading cause of death and the leading cause of disability. Produced by microscopic organisms that enter a person’s body, infectious diseases can create symptoms ranging from mild digestive problems to deadly bleeding and high fevers. You probably have heard of the top three killers: tuberculosis (TB), acquired immunodeficiency syndrome (AIDS), and malaria. But newer viruses have made headlines in recent years, including Ebola, Marburg, West Nile, and severe acute respiratory syndrome (SARS). Scientists have yet to devise a cure for some. So what causes new and re-emerging infectious diseases? How should you protect yourself? Discover the intriguing story behind these invisible invaders.