Pathogens: Meaning, Types, and Examples

The microorganisms or infectious agents that are capable of causing diseases are called pathogens. The word pathogen is derived from two Greek words: pathos means "suffering", and genes means "producer of".  The branch of science that deals with the sudy of pathogenic bacteria or microbes such as algae, virus, bacteria, etc. is called microbiology. 

• Pathogens of all types have the ability to enter their host and interact with the immune system.
• Mycobacterium tuberculosis is an example of a common pathogen that causes Tuberculosis disease. 

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What are Pathogens?

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Microorganisms that are capable of causing diseases are called pathogens. The spread of the disease depends on the effectiveness of pathogenic organisms entering the host’s body. Food-borne pathogens such as Salmonella typhi, which causes the disease typhoid in humans, can survive and thrive at temperatures between 40^oF-140^oF.

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Types of Pathogens

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The effectiveness of pathogens determines the severeness of the disease. Any infectious agent or germ are also considered as pathogen. Their number increases once they enter the host’s body. Overall, pathogenic organisms are broadly classified into five types: Algae, Bacteria, Fungi, Viruses, and Protozoa. 

Algae

Disease caused by algal pathogens is very rare. Algae are unicellular eukaryotes that grow in the presence of sunlight, carbon dioxide, and sufficient water. Chlorophyll deficiency in green plants is caused by algal pathogens. When these plants are eaten by cattle, dogs, cats, and humans, it causes a serious algal disease called protothecosis.

Bacteria

Bacteria are single celled microbes (micro-organisms). They are further categorized into two types-pathogenic and non-pathogenic bacteria. The transmission of pathogenic bacteria occurs mainly through air, water, soil, and physical contact. They affect the immune cells and release toxins into the host’s body. Bacterial infections such as typhoid, tetanus, diptheria, etc. are treated with antibiotics.

Prion

These diseases occur when normal prion protein, found on the surfaces of several cells, becomes abnormal and mass in the brain, causing brain damage. This abnormal collection of protein in the brain can cause memory impairment, personality changes, and difficulties with movement.

Plants play vital roles for prions. There are 8 different diseases that affect mammals that are caused by prions such as scrapie, bovine spongiform encephalopathy (mad cow disease) and Feline spongiform encephalopathy (FSE). Almost 10 diseases that affect humans such as Creutzfeldt–Jakob disease (CJD) and Fatal familial insomnia (FFI) etc.

Viroid

Certain pathogens are known for affecting mostly plants. Structurally they are smaller than viruses even, and are the smallest infectious pathogens known. Viroids have a composition exclusively of a short, single-stranded, circular RNA, with no protein coating. All viroids are known to cause disease as they inhabit angiosperms. Pospiviroidae and Avsunviroidae are the two known families of viroids, around 30 known viroids are within these two families. Viroids cause hepatitis in humans. The diseases that are caused by viroids in plants are potato spindle tuber, citrus exocortis, cucumber pale fruit, chrysanthemum stunt, etc.

Virus

The Virus is a microorganism that is tinier than a bacterium which cannot grow or reproduce without a living cell. When infected by a virus, a host cell is compelled to rapidly produce thousands of identical versions of the original virus. Once the reproduction cycle ends it moves to a new host. Viruses damage the host cells. Viruses travel in different forms like airborne, physical contact etc.

Some commonly known viruses are:

• Influenza
• Rotaviruses
• Measles
• Mumps
• HIV
• COVID19, SARS-CoV-2, etc.

Parasites

Parasites are of different types, Protozoan, Helminths (Parasitic worms), Endoparasites and Ectoparasites. Protozoans are single celled eukaryotes which are called as one celled animal. Most of these pathogens are considered human pathogens and cause many kinds of diseases.

• Example: Malaria, amoebiasis, giardiasis, toxoplasmosis, cryptosporidiosis, trichomoniasis, African trypanosomiasis (sleeping sickness), etc.
• Helminths are macro parasites and can be seen with naked eye. They affect the host’s digestive nutrients and immune system and cause many diseases namely pinworm infection, etc.
• Endoparasites cause infection inside the human body. Example: Roundworms, Tape worms, etc.
• Ectoparasites infect within the skin. Example: Bed bugs, Fleas, Lice, etc.

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Previous Years Questions

1. The infectious stage of Plasmodium that enters the human body… [NEET 2020]
2. In higher vertebrates, the immune system can distinguish….[NEET 2016]
3. Which of the following is a pair of viral diseases?...[NEET 2009]
4. In which disease does mosquito transmitted...[NEET 2018]
5. A person likely to develop tetanus is immunized by….[NEET 2009]
6. A person suffering from a disease caused by Plasmodium...[NEET 2010]
7. Antibodies in our body are complex..[NEET 2006]
8. Antigens are present...[NEET 1995]
9. Antivenom injection contains preformed antibodies….[NEET 2016]
10. Cancer cells are more easily damaged by radiation...[NEET 2004]
11. Cancerous cells can easily be destroyed by...[NEET 2002]
12. Carcinoma refers to...[NEET 2003]
13. Cell-mediated immunity in the human body...[NEET 2013]
14. Christmas disease is another name for...[NEET 2003]
15. Cirrhosis of liver is caused by the chronic...[NEET 2012]
16. Infection of Ascaris usually occurs by...[NEET 2013]
17. Lysozyme that is present in perspiration...[NEET 2007]
18. Which of the following diseases is now considered completely eradicated...[NEET 1997]