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From Wikipedia, the free encyclopedia

A pathogen, (from Greek πάθος pathos "suffering, passion", and γἰγνομαι (γεν-) gignomai (gen-) "I give birth to") an infectious agent, or more commonly germ, is a biological agent that causes disease to its host.[1][2] There are several substrates and pathways whereby pathogens can invade a host; the principal pathways have different episodic time frames, but soil contamination has the longest or most persistent potential for harboring a pathogen.

The body contains many natural orders of defense against some of the common pathogens (such as Pneumocystis) in the form of the human immune system and by some "helpful" bacteria present in the human body's normal flora. However, if the immune system or "good" bacteria is damaged in any way (such as by chemotherapy, human immunodeficiency virus (HIV), or antibiotics being taken to kill other pathogens), pathogenic bacteria that were being held at bay can proliferate and cause harm to the host. Such cases are called opportunistic infection.

Some pathogens (such as the bacterium Yersinia pestis, which may have caused the Black Plague, the Variola virus, and the Malaria protozoa) have been responsible for massive numbers of casualties and have had numerous effects on afflicted groups. Of particular note in modern times is HIV, which is known to have infected several million humans globally, along with the Influenza virus. Today, while many medical advances have been made to safeguard against infection by pathogens, through the use of vaccination, antibiotics, and fungicide, pathogens continue to threaten human life. Social advances such as food safety, hygiene, and water treatment have reduced the threat from some pathogens.

Not all pathogens are negative. In entomology, pathogens are one of the "Three P's" (predators, pathogens, and parasitoids) that serve as natural or introduced biological controls to suppress arthropod pest populations.

Below is a list of different types of notable pathogens as categorized by their structural characteristics, and some of their known and predicted effects on infected host (person).




Pathogenic viruses are mainly those of the families of: Adenoviridae, Picornaviridae, Herpesviridae, Hepadnaviridae, Flaviviridae, Retroviridae, Orthomyxoviridae, Paramyxoviridae, Papovaviridae, Polyomavirus, Rhabdoviridae, Togaviridae. Some notable pathogenic viruses cause: smallpox, influenza, mumps, measles, chickenpox, ebola, and rubella. Viruses typically range between 20-300 nanometers in length.


Although the vast majority of bacteria are harmless or beneficial, a few pathogenic bacteria can cause infectious diseases. The most common bacterial disease is tuberculosis, caused by the bacterium Mycobacterium tuberculosis, which affects about 2 million people mostly in sub-Saharan Africa. Pathogenic bacteria contribute to other globally important diseases, such as pneumonia, which can be caused by bacteria such as Streptococcus and Pseudomonas, and foodborne illnesses, which can be caused by bacteria such as Shigella, Campylobacter and Salmonella. Pathogenic bacteria also cause infections such as tetanus, typhoid fever, diphtheria, syphilis and leprosy. Bacteria can often be killed by antibiotics. They typically range between 1-5 micrometers in length.


Fungi comprise a eukaryotic kingdom of microbes that are usually saprophytes but can cause diseases in humans, animals and plants. Fungi are the most common cause of diseases in crops and other plants. Life threatening fungal infections in humans most often occur in immunocompromised patients or vulnerable people with a weakened immune system, although fungi are common problems in the immunocompetent population as the causative agents of skin, nail or yeast infections. Most antibiotics that function on bacterial pathogens cannot be used to treat fungal infections due to the fact that fungi and their hosts both have eukaryotic cells. Most clinical fungicides belong to the azole group. The typical fungal spore size is 1-40 micrometer in length.

Eukaryotic, other than fungal

Some eukaryotic organisms, such as protists and helminths, cause disease. One of the best known diseases caused by protists in the genus Plasmodium is malaria.


Prions are infectious pathogens that do not contain nucleic acids. Protein malformations caused by prion infections are implicated in scrapie, bovine spongiform encephalopathy (mad cow disease) and Creutzfeldt–Jakob disease.[3]


One hypothesis regarding pathogens states that the longer a pathogen can survive outside of the body, the more dangerous it can be to a potential host.[citation needed] For example, the smallpox virus (variola virus) can survive outside the human body for approximately 885 days. It is also one of the most deadly pathogenic viruses, as it kills between 20-50% of the people it infects. The tuberculosis bacterium kills 1 in 5 of the people it infects, but only survives 244 days outside of its host. However, research into the basis of the ability of pathogens to cause disease provides evidence from multiple and diverse species of the existence of pathogenicity or virulence factors, encoded within the pathogens' genetic material, that facilitate microbes to cause disease.

In countries that have higher sanitation standards, pathogens cannot survive for as long outside of the human. This is seen as encouragement to mutations to the pathogen which would make it less deadly, as such mutations would allow the pathogen to survive in the host for longer periods of time.[citation needed]


One of the primary pathways by which food or water become contaminated is from the release of untreated sewage into a drinking water supply or onto cropland, with the result that people who eat or drink contaminated sources become infected. In developing countries most sewage is discharged into the environment or on cropland as of 12 August 1985[citation needed]; even in developed countries there are periodic system failures resulting in a sanitary sewer overflow.

Examples of pathogens

Major human pathogens

See also


External links

Study guide

Up to date as of January 14, 2010
(Redirected to Getting to know pathogens article)

From Wikiversity

Pathogenic Leptospires spirochetes can cause high fever, severe headache, chills, muscle aches, and vomiting, abdominal pain, diarrhea, or a rash.

Welcome to Getting to know pathogens. This is an introduction to basic concepts related to disease-causing micro-organisms.

Chapter readings are from "Medical Microbiology" by P. R. Murray, K. S. Rosenthal, G. S. Kobayashi and M. A. Pfaller (4th Edition) Mosby Inc. 2002 ISBN: 0323012132. You can probably find used copies for a low price.

Another option is "Medical Microbiology" (Samuel Baron, editor) which is available online (free) or as a CD-ROM] ($10).


Host-microbe interactions

The interaction of humans with disease-causing bacteria is often thought of in terms of a host-invader interaction. However, there are many types of human-microbe interactions, so we need a more complex understanding of micro-organisms and their roles in normal human health and disease processes. It is useful to think in evolutionary terms. The various types of human-microbe interactions are the result of hundreds of millions of years of interactions between animals and micro-organisms.

Host Susceptibility

  • Reading (Chapter 1)
Syphilitic rash due to the systemic spread of the Treponema pallidum bacteria
Human immunodeficiency virus (HIV).

Strict pathogens

Some (relatively few) microbes can infect essentially all human hosts who are exposed to the particular microbe AND cause essentially the same sort of infection and disease symptoms in every infected person. “Strict pathogens”

  • examples: Treponema pallidum (syphilis); HIV (AIDS); Plasmodium vivax (malaria)
Plasmodium lifecycle PHIL 3405 lores.jpg


  1. What percent of people who are infected with Treponema pallidum show symptoms of syphilis?
  2. Does HIV infection always cause AIDS?
  3. How does genetic background influence the incidence and severity malaria?

Opportunistic pathogens

Most microbes are more efficient at infecting some people than others and many microbes can cause several different types of disease depending on the type of infection (for example, depending on the site of infection) and variations in host-microbe interactions. “Opportunistic pathogens”

  • example, Escherichia coli
    • Discussion. Escherichia coli is a very common type of bacteria that normally lives in close contact with humans. Under what special circumstances can Escherichia coli cause human disease?

Human variability

We often speak of "the human body" and "human-microbe interactions" but it is wrong to think that all people have similar interactions with micro-organisms. It is important to keep in mind the full spectrum of human variability.

Sources of variation in host susceptibility to microbes:

  • variation in nutritional status,
  • variation in levels of stress and circulating stress hormones,
  • variation in genes that confer resistance to microbes,
  • variations in somatic cell mutations involved in immune system function,
  • physical damage to tissues can open tissue barriers that normally limit microbial infections.
  • Behavioral differences. Some behaviors promote health and avoid pathogens, other behaviors damage defenses and bring people into contact with pathogenic microbes.
    • Example: intravenous drug use allows exposure to disease-causing microbes.

General Properties of Infections

  • Reading (Chapter 9)

normal microflora

Lactobacillus cells associated with a human cell.

Many microbial infections are called “endogenous infections” because they are caused by microbes that normally are found on the surface of human hosts.

  • Types of colonization-
    • not harmful:
      • Transient colonization.
      • Permanent colonization.

Each surface of the body has its own population of typical microbes.

Clostridium difficile cells.

Often, these are protective against harmful microbes.

    • harmful, disease causing:
      • damage to the host by the microbe
      • disruption of normal host tissue functions due to immune system response to an otherwise harmless microbe.

example: Lactobacillus species are generally harmless GI tract microbes. Some antibiotics reach high levels in the GI tract and can greatly reduce or eliminate the normal gut microbes. Opportunistic disease-causing organisms such as Clostridium difficle can then proliferate and cause disease.

Discussion Topic

Prospects for maintaining normal (protective) microbial flora.

  • a. Diet.
  • b. Replacement of bacteria after exposure to antibiotics.
Aspergillus (fungal organism).

Example: Aspergillosis is caused by a fungus (Aspergillus), which is found commonly growing on dead leaves, stored grain, compost piles, or other decaying vegetation. Healthy humans are not susceptible to Aspergillus. Those at risk for Aspergillosis include immunosuppressed people such as organ transplant recipients, people being treated for cancer, and AIDS patients Aspergillus can also cause illness by colonization and growth in a lung cavity that was damaged by previous disease such as tuberculosis. When growing in the lung it produces a fungus ball called aspergilloma. The lung colonization can also progress to clinical pneumonia and invasive infection that is spread to other parts of the body by the blood stream (Pulmonary aspergillosis; invasive type).


Josh Wright: 19th October 2006 10:24 EAST.

One day, the whole world will die from virus's. The only help, people who study them, and know how to treat, a research antivaccines to cure them. I think that we need to put more effort into researching virus's, and pathogens, and then we will one day be able to save the world from complete death.

Are viruses all bad? Much of the genetic difference between humans and chimps has come about by way of insertions and deletions. A significant portion of human genetic change is due to the large amount of virus-derived sequence in the human genome (see). In addition to playing an important role in the rate of evolution of existing species, some hypotheses suggest that viruses played a fundamental role in the origin of cellular life as we know it; see: Three RNA cells for ribosomal lineages and three DNA viruses to replicate their genomes: A hypothesis for the origin of cellular domain by Patrick Forterre. --JWSchmidt 12:57, 19 October 2006 (UTC)

Simple English

File:EM of influenza
Influenza viruses are common pathogens. This image has a magnification of about 100.000 times

A pathogen (which is also called infectious agent) is a biological agent (like a virus, bacteria, fungi or parasite) that causes a disease in its self

Pathogenic bacteria: these are bacteria that cause disease in humans, animals and plants. Some bacteria can only make a single host ill; however, others can cause problems for a number of hosts. This all depends on the host specificity of that particular pathogenic bacterium. The diseases and other physical problems are as diverse as the pathogens that cause them and can range from: food poisoning to tooth ache and even some types of cancer.a pathogen may be fungi bacteria virus protozoaPathogenic fungi: these are fungi that exhibit pathogenic qualities however; the fungi are split into two different kinds of pathogens, there are superficial ones, which are opportunistic and cannot survive inside the human body but instead cause problems like athlete’s foot. But the select few that can survive within the human body are known as the true pathogens

Pathogenic bacteria are often found in the synthetic grass used to construct sports fields. This is a major problem, because the fields need to be regularly disinfected because the bacteria can not be broken down by any natural resources.

Pathogenic bacteria can be found in car tires. host.


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