Pathogens

Pathogens can be spread from person to person in a number of ways. Not all pathogens use all the available routes. For example, the influenza virus is transmitted from person to person through the air, typically via sneezing or coughing. But the virus is not transmitted via water. In contrast, Escherichia coli is readily transmitted via water, food, and blood, but is not readily transmitted via air or the bite of an insect.

While routes of transmission vary for different pathogens, a given pathogen will use a given route of transmission. This has been used in the weaponization of pathogens. The best-known example is anthrax. The bacterium that causes anthrax—Bacillus anthracis—can form an environmentally hardy form called a spore. The spore is very small and light. It can float on currents of air and can be breathed into the lungs, where the bacteria resume growth and swiftly cause a serious and often fatal form of anthrax. As demonstrated in the United States in the last few months of 2001, anthrax spores are easily sent through the mail to targets. As well, the powdery spores can be released from an aircraft. Over a major urban center, modeling studies have indicated that the resulting casualties could number in the hundreds of thousands.

Contamination of water by pathogens is another insidious route of disease spread. Water remains crystal clear until there are millions of bacteria present in each milliliter. Viruses, which are much smaller, can be present in even higher numbers without affecting the appearance of the liquid. Thus, water can be easily laced with enough pathogens to cause illness.

Food borne pathogens cause millions of cases of disease and hundreds of deaths each year in the United States alone. Frequently the responsible microbes are bacteria, viruses, or protozoa that usually reside in the intestinal tract of humans or other creatures. Examples of microorganisms include Escherichia coli O157:H7, Campylobacter jejuni, and rotavirus.

Pathogens can be transmitted to humans through contact with animals, birds, and other living creatures that naturally harbor the microorganism. The agent of anthrax—Bacillus anthracis—naturally dwells in sheep. Other examples include Brucella abortic (Brucellosis), Coxiella burnetti (Q fever), and viruses that cause hemorrhagic fevers such as Ebola and Marburg.