ABSTRACT
Infectious diseases have plagued humans since the Neolithic Agricultural Revolution. At that time communities became compactly settled, grew in size, and interacted closely with domestic animals and each other. Each of these factors continues to play a role in the presence and spread of communicable diseases. The terms “infectious,” “communicable,” and “contagious” signal the fact that certain human diseases are transmitted among humans. Many diseases, of course, are not. One does not “catch” obesity, diabetes, cancer, or arthritis by proximity to, or contact with, other people. One does, however, “catch” a cold, measles, plague, cholera, or influenza from contact with, or the proximity of, others whose bodies carry the appropriate pathogens – e.g. bacteria or viruses. Some infectious diseases require direct contact of two people; HIV/AIDS and syphilis are examples. Others are communicated through the air (influenza or primary pneumonic plague), water (cholera), or tainted surfaces (common cold) touched by two or more people. Still others require living vectors to move between the human carrier and the new victim. Mosquitos transmit malaria, body lice typhus, and certain fleas bubonic plague. A number of human disease outbreaks actually originate among animal populations and then spread to human communities. For example, plague’s rat fleas prefer living within rat fur, but when the diseased rodent dies its infected fleas seek a new home. Since rats, especially the black rattus rattus, enjoy human company, the closest new hosts are often people and domestic animals, and so the disease jumps species. As this piece is being written, 50,000,000 chickens in the American Midwest are reportedly being slaughtered out of fear of their having or spreading a form of “bird flu” virus that can be and has been transmitted to human populations (Revkin 2015).
