The lessons of pandemics throughout history have shaped how public health addresses emerging infectious disease threats. Pathogenic microorganisms cause infectious diseases that spread by direct or indirect transmission.

If an environment is favorable for the infectious agent’s survival and opportunity exists for the host’s exposure to it, infection and disease will ensue. Some infectious disease outbreaks result from direct contact with an infected individual’s respiratory secretions, while others are transmitted through blood or bodily fluids.

disease — communicable if the infected host has a portal of exit, a means of transmission, and a portal of entry into another susceptible host. infectious disease — disease caused by pathogenic microorganisms such as bacteria, viruses, parasites, or fungi; enters a human host, multiplies, and causes infection that can spread to others. infectious — easily transmitted between individuals. pandemic — an epidemic that spreads worldwide.