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Pathogens are microscopic organisms that cause or have the potential to cause disease. Different types of pathogens include bacteria, viruses, protists ( amoeba, plasmodium, etc.), fungi, parasitic worms (flatworms and roundworms), and prions. While these pathogens cause a variety of illness ranging from minor to life-threatening, it is important to note that not all microbes are pathogenic. In fact, the human body contains thousand of species of bacteria, fungi, and protozoa that are part of its normal flora. These microbes are beneficial and important for proper operation of biological activities such as digestion and immune system function. They only cause problems when they colonize locations in the body that are typically kept germ-free or when the immune system is compromised. In contrast, truly pathogenic organisms have a single goal: survive and multiply at all cost. Pathogens are specially adapted to infect a host, bypass the host's immune responses, reproduce within the host, and escape its host for transmission to another host.

Pathogens can be transmitted either directly or indirectly. Direct transmission involves the spread of pathogens by direct body to body contact. Direct transmission can occur from mother to child as exemplified with HIV, Zika, and syphilis. This type of direct transmission (mother-to-child) is also known as vertical transmission. Other types of direct contact through which pathogens can be spread include touching ( MRSA), kissing (herpes simplex virus), and sexual contact (human papillomavirus - HPV). Pathogens can also be spread by indirect transmission, which involves contact with a surface or substance that is contaminated with pathogens. It also includes contact and transmission through an animal or an insect vector. Types of indirect transmission include:

Airborne - pathogen is expelled (typically by sneezing, coughing, laughing, etc.), remains suspended in air, and is inhaled by or comes in contact with respiratory membranes of another person.

Droplets - pathogens contained in droplets of body fluid (saliva, blood, etc.) contact another person or contaminate a surface. Saliva droplets are most commonly spread through sneezing or coughing.

Foodborne - transmission occurs through eating contaminated food or by improper cleaning habits after handling contaminated food.

Waterborne - pathogen is spread by consumption or contact with contaminated water.

Zootonic - pathogen is spread from animals to humans. This includes insect vectors that transmit disease through biting or feeding and transmission from wild animals or pets to humans.

While there is no way to completely prevent pathogen transmission, the best way to minimize the chances of acquiring a pathogenic disease is by maintaining good hygiene. This includes washing your hands properly after using the restroom, handling raw foods, handling pets or pet excrement, and when coming in contact with surfaces that have been exposed to germs.

Types of Pathogens

Pathogens are very diverse and consist of both prokaryotic and eukaryotic organisms. The most commonly known pathogens are bacteria and viruses. While both are capable of causing infectious disease, bacteria and viruses are very different. Bacteria are prokaryotic cells that cause disease by producing toxins. Viruses are particles of nucleic acid (DNA or RNA) encased within a protein shell or capsid. They cause disease by taking over their host's cell machinery to make numerous copies of the virus. This activity destroys the host cell in the process. Eukaryotic pathogens include fungi, protozoan protists, and parasitic worms.

A prion is a unique type of pathogen that is not an organism at all but a protein. Prion proteins have the same amino acid sequences as normal proteins but are folded into an abnormal shape. This altered shape makes prion proteins infectious as they influence other normal proteins to spontaneously take on an infectious form. Prions typically affect the central nervous system. They tend to clump together in brain tissue resulting in neuron and brain deterioration. Prions cause the fatal neurodegenerative disorder Creutzfeldt-Jakob disease (CJD) in humans. They also cause bovine spongiform encephalopathy (BSE) or mad cow disease in cattle.

Bacteria are responsible for a number of infections that range from asymptomatic to sudden and intense. Diseases brought on by pathogenic bacteria are commonly the result of the production of toxins. Endotoxins are components of the bacterial cell wall that are released upon the death and deterioration of the bacterium. These toxins cause symptoms including fever, blood pressure changes, chills, septic shock, organ damage, and death.

Exotoxins are produced by bacteria and released into their environment. Three types of exotoxins include cytotoxins, neurotoxins, and enterotoxins. Cytotoxins damage or destroy certain types of body cells. Streptococcus pyogenes bacteria produce cytotoxins called erythrotoxins that destroy blood cells, damage capillaries, and cause the symptoms associated with flesh-eating disease. Neurotoxins are poisonous substances that act on the nervous system and brain. Clostridium botulinum bacteria release a neurotoxin that causes muscle paralysis. Enterotoxins affect cells of the intestines causing severe vomiting and diarrhea. Bacterial species that produce enterotoxins include Bacillus, Clostridium, Escherichia, Staphylococcus, and Vibrio.
Pathogenic Bacteria

Viruses are unique pathogens in that they are not cells but segments of DNA or RNA encased within a capsid (protein envelope). They cause disease by infecting cells and commandeering cell machinery to produce more viruses at a rapid rate. They counter or avoid immune system detection and multiply vigorously within their host. Viruses not only infect animal and plant cells, but also infect bacteria and archaeans.

Viral infections in humans range in severity from mild (cold virus) to lethal (Ebola). Viruses often target and infect specific tissues or organs in the body. The influenza virus, for example, has an affinity for respiratory system tissue resulting in symptoms that make respiration difficult. The rabies virus commonly infects central nervous system tissue, and the various hepatitis viruses home in on the liver. Some viruses have also been linked to the development of some types of cancer. Human papilloma viruses have been linked to cervical cancer, hepatitis B and C have been linked to liver cancer, and the Epstein-Barr virus has been linked to Burkitt's lymphoma ( lymphatic system disorder).
Pathogenic Viruses

Ebola virus: Ebola virus disease, hemorrhagic fever

Human Immunodeficiency Virus (HIV): pneumonia, sinus infections, meningitis

Influenza virus: flu, viral pneumonia

Norovirus: viral gastroenteritis (stomach flu)

Varicella-zoster virus (VZV): chickenpox

Zika virus: Zika virus disease, microcephaly (in infants)

Fungi are eukaryotic organisms that include yeast and molds. Disease caused by fungi is rare in humans and typically the result of a breach of a physical barrier ( skin, mucus membrane lining, etc.) or compromised immune system. Pathogenic fungi often cause disease by switching from one form of growth to another. That is, unicellular yeasts exhibit reversible growth from yeast-like to mold-like proliferation, while molds switch from mold-like to yeast-like growth.

The yeast Candida albicans changes morphology by switching from round budding cell growth to mold-like elongated cell (filamentous) growth based on a number of factors. These factors include changes in body temperature, pH, and the presence of certain hormones. C. albicans causes vaginal yeast infections. Similarly, the fungus Histoplasma capsulatum exists as filamentous mold in its natural soil habitat but switches to budding yeast-like growth when inhaled into the body. The impetus for this change is increased temperature within the lungs as compared to soil temperature. H. capsulatum causes a type of lung infection called histoplasmosis that can develop into lung disease.
Pathogenic Fungi

Protozoa

Protozoa are tiny unicellular organisms in the Kingdom Protista. This kingdom is very diverse and includes organisms such as algae, euglena, amoeba, slime molds, trypanosomes, and sporozoans. The majority of protists that cause disease in humans are protozoans. They do so by parasitically feeding off of and multiplying at the expense of their host. Parasitic protozoa are commonly transmitted to humans through contaminated soil, food, or water. They can also be transmitted by pets and animals, as well as by insect vectors.

The amoeba Naegleria fowleri is a free-living protozoan found commonly in soil and freshwater habitats. It is termed the brain-eating amoeba because it causes the disease called primary amebic meningoencephalitis (PAM). This rare infection commonly occurs when individuals swim in contaminated water. The amoeba migrates from the nose to the brain where it damages brain tissue.
Pathogenic Protozoa

Parasitic worms infect a number of different organisms including plants, insects, and animals. Parasitic worms, also called helminths, include nematodes ( roundworms) and platyhelminthes ( flatworms). Hookworms, pinworms, threadworms, whipworms, and trichina worms are types of parasitic roundworms. Parasitic flatworms include tapeworms and flukes. In humans, the majority of these worms infect the intestines and sometimes spread to other areas of the body. Intestinal parasites attach to the walls of the digestive tract and feed off of the host. They produce thousands of eggs that hatch either inside or outside (expelled in feces) of the body.

Parasitic worms are spread through contact with contaminated food and water. They can also be transmitted from animals and insects to humans. Not all parasitic worms infect the digestive tract. Unlike other Schistosoma flatworm species that infect the intestines and cause intestinal schistosomiasis, Schistosoma haematobium species infect the bladder and urogenital tissue. Schistosoma worms are called blood flukes because they inhabit blood vessels. After the females lay their eggs, some eggs exit the body in urine or feces. Others may become lodged in body organs ( liver, spleen, lungs) causing blood loss, colon obstruction, enlarged spleen, or excessive fluid buildup in the abdomen. Schistosoma species are transmitted by contact with water that has been contaminated with Schistosoma larvae. These worms enter the body by penetrating the skin.
Pathogenic Worms
References

Alberts B, Johnson A, Lewis J, et al. Molecular Biology of the Cell. 4th edition. New York: Garland Science; 2002. Introduction to Pathogens. Available from: https://www.ncbi.nlm.nih.gov/books/NBK26917/

Kobayashi GS. Disease of Mechanisms of Fungi. In: Baron S, editor. Medical Microbiology. 4th edition. Galveston (TX): University of Texas Medical Branch at Galveston; 1996. Chapter 74. Available from: https://www.ncbi.nlm.nih.gov/books/NBK8103/

Relevant Pathogens from A to Z. (n.d.). Retrieved March 22, 2017, from http://www.bode-science-center.com/center/relevant-pathogens-from-a-z.html

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What Are the Different Types of Pathogens?

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Pathogens are microscopic organisms that cause or have the potential to cause disease. Different types of pathogens include bacteria, viruses, protists ( amoeba, plasmodium, etc.), fungi, parasitic worms (flatworms and roundworms), and prions. While these pathogens cause a variety of illness ranging from minor to life-threatening, it is important to note that not all microbes are pathogenic.

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Types of Pathogens | A-level Biology | OCR, AQA, Edexcel