Microorganisms that cause disease in humans and other species are known as pathogens. The transmission of pathogens to a human or other host can occur in a number of ways, depending upon the microorganism.
A common route is via water. The ingestion of contaminated water introduces the microbes into the digestive system. Intestinal upsets can result. As well, an organism may be capable of entering the cells that line the digestive tract and gaining entry to the bloodstream. From there, an infection can become widely dispersed. A prominent example of a water borne pathogen is Vibrio cholerae, the bacterium that causes cholera. The contamination of drinking water by this bacterium is still at epidemic proportions in some areas of the world.
Pathogens can also be transmitted via the air. Viruses and bacterial spores are light enough to be lifted on the breeze. These agents can subsequently be inhaled, where they cause lung infections. An example of such as virus is the Hanta virus. A particularly prominent bacterial example is the spore form of the anthrax-causing bacterium Bacillus anthracis. The latter has also been identified as a bioterrorist weapon that can, as exemplified in a 2001 terrorist attack on the United States, be transmitted in mail that when opened or touched can result in cutaneous or inhalation anthrax.
Still other microbial pathogens are transmitted from one human to another via body fluids such as the blood. This route is utilized by a number of viruses. The most publicized example is the transmission of Human Immunodeficiency Virus (HIV). HIV is generally regarded to be the cause of acquired immunodeficiency syndrome. As well, viruses that cause hemorrhagic fever (e.g., Ebola) are transmitted in the blood. If precautions are not taken when handling patients, the caregiver can become infected.
Transmission of pathogens can occur directly, as in the above mechanisms. As well, transmission can be indirect. An intermediate host that harbors the microorganism can transfer the microbes to humans via a bite or by other contact. Coxiella burnetti, the bacterium that cause Q-fever, is transmitted to humans from the handling of animals such as sheep. As another example, the trypanosome parasite that causes sleeping sickness
Finally, some viruses are able to transmit infection over long periods of time by become latent in the host. More specifically, the genetic material of viruses such as the hepatitis viruses and the herpes virus can integrate and be carried for decades in the host genome before the symptoms of infections appear.
See also Anthrax, terrorist use as a biological weapon; Bacteria and bacterial infection; Epidemics and pandemics; Yeast, infectious; Zoonoses
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