1. Infectious diseases are caused by microorganisms like bacteria, viruses, fungi or parasites that can be passed from person to person. They remain a leading cause of death worldwide. 2. The document discusses the routes of transmission for infectious agents, including direct contact, airborne transmission, and environmental survival of the infectious agent outside a host. Factors like these determine how quickly and widely an agent can spread. 3. Major infectious disease pandemics and epidemics throughout history have had significant social and economic impacts. The COVID-19 pandemic demonstrates how easily infections can spread globally in the modern, interconnected world.