Pathogen
From ArticleWorld
A pathogen is an infections agent, such as a virus, bacterium, protozoan, fungus, parasite or protein that causes disease states in a host organism. In humans, infections from all of these types of agents are likely.
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Pathogen versus non-pathogen
It is important to know that not all microorganisms are pathogenic. The human body is host to a number of unharmful, symbiotic bacteria and fungi that live in the digestive tract, on the skin and in parts of the genitourinary tract. No symptoms or disease results from these microorganisms with the exception of those that cause opportunistic infections. Opportunists are nonpathogenic unless they are allowed to take over parts of the body where they are not normally found or if they overgrow in parts of the body.
Bacteria and viruses
There are numerous types of pathogenic bacteria and viruses. E. coli, Staphylococcus and Streptococcus strains are sometimes pathogens. Viruses that cause influenza, hepatitis viruses and rhinoviruses are also almost always disease-causing. Viruses are basically encapsulated particles that contain DNA or RNA and the ability to infect a cell and replicate itself using the cell’s energy machinery. Viruses cannot live long outside of a host.
Bacteria are single-celled organisms that can divide and replicate themselves with or without a host. They also have their own machinery to survive if given the proper nutrition. Some are not pathogenic at all. Others are almost always pathogenic. Bacteria and viruses both tend to focus on a particular body area to infect. For example, rhinoviruses infect the mucus membranes of the upper airway and Helicobacter pylori infects the stomach, causing stomach ulcers.
Fungi and protozoans
These organisms also have species that live independently and do not cause disease in humans. Certain species, however, are pathogenic. The pathogenic species tend to be single-celled and are transferred via contact, water or, in some cases, a vector like mosquitoes. The protozoan, Plasmodium, is transferred to humans via a mosquito bite and causes the disease known as malaria. Giardia lamblia is another single-celled protozoan that is often transferred to humans via infected water. Fungal organisms such as Candida and Tinea species infect primarily the skin; however, Candida can infect the urogenital tract, the mouth and the intestinal tract.
Parasites and proteins
These two categories represent the opposite ends of the spectrum when it comes to the complexity of pathogens. Parasites can be as big as tapeworms that are several feet long to lice and pinworms, which are very tiny. Prions are essentially pathogenic proteins. They have no real cellular shape or machinery and are less complex than viruses. Prions cause diseases like mad cow disease, or bovine spongiform encephalopathy, and are transferred by direct contact with an animal that is infected.