Pathogens are microorganisms such as viruses, bacteria, protozoa, and fungi that cause disease in humans and other species. Pathogen transmission involves three steps: escape from the host, travel to, and infection of the new host. Pathogen transmission occurs in several ways, usually dependent on the ecology of the organism. For example, respiratory pathogens are usually airborne, while pathogens of the digestive tract tend to occur in food or water. Epidemiologists group pathogen transmission into two general types of contact, direct and indirect, within which there are several mechanisms.
Pathogen transmission by direct contact takes place when an infected host transmits a disease directly to another host. The pathogens that travel this way are extremely sensitive to the environment and cannot be outside of the host for any length of time. For example, pathogens that cause sexually transmitted diseases (STDs) are transmitted via blood, semen, or saliva. Some pathogens responsible for STDs include Tremonema palidum (syphilis), Neisseria gohorrhoeae (gonorrhea), and the pathogen that causes Acquired Immunodeficiency Syndrome or AIDS, Human Immunodeficiency Virus (HIV). The viruses responsible for hemorrhagic fever, such as Ebola, are also transmitted by direct contact via the blood.
Indirect transmission occurs when an agent is required to transfer the pathogen from an infected host to a susceptible host. The agent may be either animate or inanimate. Animate transmission agents, which are referred to as disease vehicles, include air, water, and food. Inanimate agents also include fomites, which are objects on which the pathogen has been deposited. Examples of fomites are toys, clothes, bedding, or surgical instruments. Animate, or living, agents of disease transmission are most often insects, mites, fleas, and rodents. Living agents of transmission are referred to as vectors. Diseases that are spread via indirect contact in hospitals are specifically referred to as nosocomial infections.
Many respiratory viruses and bacterial spores are light enough to be lifted by the wind. These agents can subsequently be inhaled, where they cause lung infections. A particularly important example of an airborne bacterial pathogen is the spore form of the anthrax-causing bacterium Bacillus anthraci . This bacterium forms spores that can spread through the air and cause a severe respiratory disease when inhaled. Biological weapons can be equipped with anthrax spores aimed at infecting populations upon detonation. In 2001, the United States was plagued by a bioterrrorist who placed spores in mail so that the people who handled the envelopes contracted cutaneous or inhalation anthrax.
A common route of indirect pathogen transmission is via water. The ingestion of contaminated water introduces the microbes into the digestive system, where they can attack the gastrointestinal tract. Some pathogenic organisms use the cells that line the digestive tract in order to gain entry to the bloodstream. From there, an infection can become systemic. A common water borne pathogen is Vibrio cholerae , the bacterium that causes cholera. The contamination of drinking water by this bacterium still causes cholera epidemics in some areas of the world.
Foodborne pathogens are grouped into two categories: those that produce toxins that poison the host and those that infect the host and then grow there. Food poisoning is most often caused by the bacterium Staphylococcus aureus , which produces enterotoxins that result in vomiting and diarrhea. The bacterium Clostridium botulinum is responsible for the disease botulism, which is an extremely severe and sometimes fatal food poisoning.
Vectors harbor the microorganisms that cause disease and transfer them to humans via a bite or by other contact. Coxiella burnetti , the bacterium that causes Q fever, is transmitted to humans from the handling of animals such as sheep. Insects are common vectors of disease. Mosquitos spread the protozoan Plasmodium vivax that causes malaria. Deer ticks are responsible for infection by the spirochete Borrelia burgdorferi that causes Lyme disease. The bacterium that causes plague Yersina pestis is transmitted by the rat flea.
According to the United States Centers for Disease Control, the pathogens that are most likely to be used as biological weapons use a variety of modes of transmission. Included in this list of pathogens are the airborne bacterium Bacillus anthracis and the airborne Variola virus that causes smallpox; the foodborne bacterium Clostridium botulinum ; Yersina pestis , which requires a vector; and the Ebola virus, which requires direct bloodborne transmission.
█ FURTHER READING:
United States Centers for Disease Control. "Biological Diseases/Agents." < http://www.bt.cdc.gov/Agent/agentlist.asp#categorydescriptions > (February 26, 2003).