Pursue him to his house, and pluck him thence;
Lest his infection, being of a catching nature,
Spread further.

—William Shakespeare, Coriolanus, 1608

Infectious (contagious) diseases are caused by microorganisms—viruses, bacteria, parasites, or fungi—transmitted from one person to another through casual contact, such as influenza; through bodily fluids, such as HIV (human immunodeficiency virus); or via contaminated food, air, or water supplies. Infectious diseases also may spread by vectors of disease such as insects or arthropods that carry the infectious agent.

Infectious diseases are a leading cause of death worldwide, according to the World Health Organization (WHO). Not long ago, the U.S. government and medical experts believed that widespread use of vaccines, antibiotics, and public health measures had effectively eliminated the public health threat of infectious diseases. Throughout the world, however, new and rare diseases are emerging, and old diseases are resurfacing. Some of...