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There are six main types of disease causing agents or pathogens: prions, viruses, bacteria, protozoans, fungi, and macroparasites.

These pathogens vary greatly in size and shape, and also in the type of diseases they cause in their host. Pathogens can be microscopic (too small to be seen with the unaided eye) or macroscopic (can be seen with the unaided eye).
Type of Pathogen
Definition
Characteristics
Example of a disease
Prions
· Infectious protein that causes the degeneration of brain tissue in mammals.
· A form of proteins that have undergone alteration form their normal shape to an abnormal shape, however, the chemical composition of the protein remains unaltered.
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· Cause degenerative neurological infections.
· Occurs mainly in nerve cells
· It converts normal protein into prion protein. Enables infection to grow
· ONLY in mammals.
· Eventually prompt a cell to burst and a free to infect other cells.
· Bursting of nerve cells leads to the holes seen in the infected brains.
· Resistent to heat and chemical agents and hence recommended to sterilise any item against a prion effective agent.
· No treatment available.
· A prion can produce more prions in brain tissue without any nucleic acid involvement, whilst a protein cannot.
· Scrapie in sheep
· BSE (Bovine Spongiform encephalitis) – Mad Cow Disease
· Creutzfeldt – Jakob in humans
Viruses
· Ultramicroscopic infectious agent that replicates itself only within cells of living hosts; a piece of nucleic acid (DNA or RNA) wrapped in a thin coat of protein).
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  • All viruses composed of nuclear material, DNA or RNA surrounded by an outer protein coat.
  • Much smaller than the smallest bacterial cell.
  • On the borderline of living and non-living this is because they lack a metabolism that would provide them with normal life characteristics for example don’t have independent reproductions.
  • Reproduce only inside another cell, a host cell, and these then results in the death of the host cell.
  • It is possible to crystallise viruses.
  • No cures for diseases caused by viruses but we can reduce there prevalence by vaccination. external image chickenpox.jpg
  • Mumps virus
  • Influenza VIRUS
  • AIDS
  • Chicken pox
Bacteria
· Prokaryotic cells that are present in the soil, air, water and in many humans body parts, such as the skin.
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  • Reproduce by binary fission so that within a short time, the host can contain thousands of bacteria.
  • Produce toxins often as waste products, which harm the host.
  • In plants, bacteria are found in the conducting and leave tissues. external image Mycosphaerella%20leaf%20spot%20of%20ohia.JPG
  • Tetanus
  • Pneumonia
  • Tuberculosis
  • Chlamydia
  • Foot Rot
  • Whooping Cough
  • Plants:
  • Crown gaal
  • Bacterial blossom blight
  • Leaf spot disease
  • Rot disease such as black leg.
Fungi
· eukaryotic organisms
· most composed of a system of microscopic tubular filaments or threads which branch to form a mycelium structure
· fungi are saprophytes (live on dead plant and animal materials)
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· produces fruiting bodies which contain thousands of spores by which the fungi reproduces
· play important role in decomposition of organic material
· use properties of yeast fungi to bake bread, brew beer and make wine
· fungi grow best in dark and moist conditions
external image aspergillosis.jpg
· Candidacies (thrush), tinea (athletes foot), Ringworm in humans
· Aspergillosis in birds
· Rusts, mildews, moulds and blights in plants
Macro parasites
· Multicellular organisms that are visible to the naked eye, also called parasites.

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· Metabolically dependent on another living organism ( the host)à for completion of life cycle
· Parasites are either endoparasites (live in host) or are ectoparasites (live on host).
· Example: tapeworms and fluke à endoparasites
· And mosquitoes, fleas, ticks, leeches, mites and lice à suck blood ectoparasites.
· Eukaryotic cells (cells enclosed with membrane) with cell walls, organelles but no chloroplasts.
· fleas
· ticks
· tapeworms
· bilharzia worms
· hydatid worms
· liver fluke
· many plant parasites, e.g. aphids
external image TapewormImg_1395.jpg
Protozoan or Protozoa
· Unicellular organisms that are usually categorised according to their locomotion.
· Can only divide within a host.

external image Paramecium.gif
· Flagellates- Use a flagellum (whip like tail)
· Ciliates- Use cilia to propel them
· Amoebae- Use pseudopods.
· Sporozoa- Reproduce by spores.
· Most protozoa reside in water where locomotion is simpler.
· Breed, move and breathe like multicellular organisms.
· Some are harmful as they result in infectious diseases.
· However, there are useful protozoa which aid in eliminating harmful bacteria and are food for other organisms such as fish.
African sleeping sickness
· Malaria
· Amoebic Dysentery
· Giardiasis
external image malaria_mosquito.jpg

References
Alford, D. HSC Biology, Pascal Press, NSW 2008.
Kinnear, J. HSC Course Biology (Edn 2), John Wiley & Sons Australia, Queensland, 2001.
Mudie, K. Heinemann Biology, Reed International Books Australia, Victoria, 2000.
Google Images


QUESTIONS:

1. DRAW A TABLE WHICH DISTINGUISHED BETWEEN PRIONS, VIRUSES, BACTERIA, PROTOZOANS, FUNGI AND MACRO-PARASITES, NAMING AN EXAMPLE OF EACH (IE, a brief version of the above).