Pathogens are basically microorganisms that are capable of causing diseases in organisms they invade and make their host. Pathogens derive their name from two Greek terms - ‘pathos' denoting disease and ‘geinen' meaning to produce. According to another view, the word ‘pathogen' has its origin in the Greek expression ‘birth of pain'. Majority of these microorganisms or pathogens are infectious, such as bacteria or viruses causing diseases. In addition, pathogens also include harmful parasites, such as protozoans and fungi. However, not all microorganisms are harmful or cause disease and hence, they are not called pathogens.
Pathogens are capable of invading their hosts though a number of conduits or substratum. However, the major conduits or means by which pathogens come in contact with their host or invade them include the following: air contact or breathing; water contact or intake or skin and animal contact/ bite or ingestion. While there are different time frames for pathogen infection by these means, actually contamination of soil has the most extended as well as relentless probability of sheltering the harmful and disease-bearing microorganisms.
In fact, the human body possesses several protections against a number of the more familiar pathogens, such as Pneumocystis, in the form of human immune system as well as by means of a number of ‘useful' bacteria present in the natural flora of the human body. Nevertheless, when the immune system or the ‘beneficial' bacteria is hurt in any manner, for instance owing to chemotherapy, antibiotics taken to eliminate other pathogens or human immunodeficiency virus (HIV), the disease bearing bacteria or pathogenic bacteria that had been staved off may breed and result in harm to the host. It may be mentioned here that such instances are known as opportunistic infections.
There are three phases concerning the spread of pathogens or disease-bearing microbes - first, they run away from their host, second, they travel or transmit to the new target and finally, they invade and subsequently contaminate the new host. Transmission or spread of pathogens takes place in a number of different manners, normally depending on the natural balance or ecology of the microorganism. For instance the pathogens that infect the respiratory system are normally transmitted by air or, in other words, are airborne. On the other hand, pathogens that infect the digestive system are usually found in water or food and the infection occurs when the victim consumes the contaminated water or food. According to epidemiologists, the transmission or spread of pathogens may be categorized into two common types - direct and indirect. And within these two types of transmission there are a number of different methods.
Direct pathogen transmission takes place when a host infected by the disease-bearing microorganism passes on the ailment to another host. Microorganisms that usually transmit directly from one host to another are said to be highly susceptible to the environmental conditions and are not able to remain outside the body of its host for a long enough period. For instance, pathogens that result in sexually transmitted disease (STDs) are those that are spread directly from the infected person to another host by means of blood, saliva or semen. Pathogens or harmful microorganisms that cause sexually transmitted disease include Tremonema palidum (causing syphilis), Neisseria gonorrhoea (causing gonorrhea) as well as the pathogen that is responsible for Acquired Immunodeficiency Syndrome (AIDS) and Human Immunodeficiency Virus (HIV). In fact, the microorganism or virus called Ebola that is responsible for the deadly viral hemorrhagic fever is also transmitted by direct contact through the blood of the infected person.
When a mediator is needed to transmit a pathogen from one host to another, it is known as indirect transmission. In such cases, the pathogen first leaves its host and harbors in a medium for sometime before invading and infecting the new host. The mediator or agent to facilitate an indirect transmission of pathogens may be a living organism or lifeless object. Inanimate agents that transmit pathogens are denoted as disease vehicles, including air, water and food. Fomites are also considered to be inanimate agents as the pathogen often harbors on them before invading their new hosts. Some common instances of such inanimate objects or substances include clothes, toys, bedding and even surgical instruments. On the other hand, the most common living organisms that help in transmission of pathogens include insects, fleas, mites and rodents. As inanimate transmitting agents are called disease vehicle, the living organisms or animate agents are known as vectors. It may be noted here that normally the ailments that get transmitted through indirect contact, especially in hospitals, are particularly known as nosocomial infections.
A number of viruses and bacterial germs that infect the respiratory tract are so light that they can be lifted by the wind and transported elsewhere without any difficulty. Such air-borne pathogens are breathed in by susceptible victims later on resulting in lung infections. In fact, a bacterial spore or germ of the anthrax variety is a common example of an air-borne pathogen called bacterium Bacillus anthraci. This bacterium develops germs or spores (dormant non-reproductive body formed by certain bacteria in response to adverse environmental conditions) that are able to distribute by means of the air and when they are breathed in by humans they are capable of resulting in acute ailments of the respiratory tract. In fact, even biological weapons of mass destruction can be prepared with anthrax spores with a view to contaminate hundreds and thousands of people when they are exploded. It may be mentioned here that at the beginning of this millennium, in 2001, a bio-terrorist led to an outbreak of an epidemic in the United States who put bacterial spores in mail with a view to infect the people who handled the envelopes with anthrax when they inhaled the spores or germs came in contact with their skin.
Water is a very common agent to pass on or spread pathogens indirectly. When people consume water contaminated by these harmful microorganisms, they enter the victim's digestive system and invade and subsequently infect the alimentary canal. What is worse is that a number of pathogens try to penetrate the cells coating the digestive tract and enter the bloodstream causing havoc to our health. When these harmful microorganisms succeed in entering the bloodstream, they can cause a systemic infection. In other words, this may result in the contamination of the entire body. Vibrio cholera, a bacterium that results in cholera, is a very widespread water-borne pathogen. Although this deadly disease has been brought under control in most regions of the world, water contaminated by this bacterium still causes cholera in some parts of the world where the hygienic conditions are below the desired standards.
Pathogens that are transmitted by means of foodstuff are categorized in two groups - the first group includes pathogens that generate contaminants that directly poison the host, while the second group comprises harmful microorganisms that contaminate the host and subsequently develop inside the victim's body. You would be amazed to known that the bacterium Staphylococcus aureus is usually responsible for most of the food poisoning cases. When this particular bacterium contaminates the food it produces entero-toxins that cause vomiting and diarrhea in the victim. Similarly, the bacterium Clostridium botulinum is the reason behind the ailment botulism - an extremely acute and, at times, fatal cases of food poisoning.
Animate agents transmitting pathogens or vectors offer an ideal shelter for the microorganisms and then transfer them to the humans either by biting them or by other means of contact. Some animals are also very familiar vectors and they harbor the bacterium Coxiella burnetti that is responsible for Q fever. This disease is transmitted to the humans when they deal with sheep. Insects are even common vectors or agents of transmitting pathogens and causing various diseases. For instance, mosquitoes are responsible for transmitting the protozoan known as Plasmodium vivax that is responsible for causing malaria. The spirochete Borrelia burgdorferi responsible for spreading Lyme disease is harbored by deer ticks (any of several ticks of the genus Ixodes that are parasitic on deer). On the other hand, rat flea transmits the bacterium that results in plague Yersina pestis.
The United States Centers for Disease Control has prepared a list of the pathogens that are most expected to be utilized as biological weapons and stated that the biological weapons are able to spread these pathogens by several different methods. The list of pathogens prepared by this organization include air-borne virus Variola that is responsible for smallpox and air-borne bacterium bacillus anthracis. In addition, the US Centers for Disease Control has warned that food-borne bacterium Clostridium botulinum, the Ebola virus transmitted directly by blood contact and Yersina pestis that needs a vector for transmission are also very likely to be used in biological weapons.