Infectious Diseases - Pathogens

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  • Infectious Diseases: Pathogens
  • Bacteria
  • Fungi
  • Viruses
  • Protozoa
  • Parasites
  • How pathogens are spread
  • How pathogens cause disease
  • Growth of pathogen populations
  • Quiz

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Protozoa are large, single-celled organinsms. Perhaps the most famous protozoa, the amoeba, can be up to 1 mm in size (gigantic for a single cell).

The protozoa that cause parasitic infections in humans are usually in the size range of 1-100 micrometres (millionths of a metre) and have two stages in their life cycle. Infection is usually spread by structures called cysts which are cells that have secreted a protective layer around them so that they can survive the journey from one host to the next. Once inside the new host, these cysts develop into the active protozoa which grow, reproduce and cause symptoms of disease in the new host.

Protozoa cause a number of human diseases. The best known is malaria, but they also cause one form of dysentery and also sleeping sickness.

Not all protozoa are harmful to humans. Modern sewage treatment works actually use some protozoa to help digest the raw sewage so that it can be dispersed safely into the sea.

The structure of protozoa

There are many different types of protozoa and some of them have very complex structures. But all of the protozoa have some structures in common. These include:

  • Cell membrane
    surrounds the cytoplasm and controls the movement of substances into and out of the cell
  • Nucleus
    contains the genetic material which controls the cell activities and reproduction
  • Cytoplasm
    liquid gel in which many of the chemical reactions of the cell take place
  • Vacuole
    membrane lined, fluid filled space in the cytoplasm which may be used for water balance or feeding
An amoeba – a simple protozoan

An amoeba – a simple protozoan

Medicine that acts against bacterial infections. Penicillin is an example of an antibiotic.
Protein that is produced by lymphocytes (white blood cells) and that attaches to a specific antigen.
Molecule on the surface of a pathogen that identifies it as a foreign invader to the immune system.
Single-celled organism. Has a cell wall, cell membrane, cytoplasm. Its DNA is loosely-coiled in the cytoplasm and there is no distinct nucleus.
The use of biological organisms or enzymes to create, break down or transform a material
To cut apart, or separate, tissue especially for anatomical study.
Exponential growth
If something is growing exponentially the larger the quantity gets, the faster it grows
Micro-organism that can grow in long tubes called hyphae or as single cells. Fungi have a nucleus, cytoplasm and a cell wall.
Herd immunity
If a high percentage of a population is immune to a disease the disease cannot be passed on because it cannot find new hosts.
Infection caused by the human immunodeficiency virus (HIV). It attacks and destroys the immune system.
Hybridoma cells are formed by fusing a specific antibody-producing cell with a type of cancer cell that grows well in tissue culture
Immune system
The body's natural defence mechanism against infectious diseases.
A process which gives immune resistance to a particular disease. The human or animal is exposed to a harmless antigen in order to raise antibodies and provide an immune memory.
A type of white blood cell that make antibodies to fight off infections.
A type of white blood cell that consumes dead pathogens that have been killed by antibodies.
Organism that feeds off another living host and causes it some damage. An example of a parasite is a tapeworm that lives in the digestive system of a host organism.
A micro-organism that causes disease.
Phagocytes are the white blood cells that protect the body by ingesting harmful foreign particles, bacteria, and dead or dying cells.
A polymer made up of amino acids joined by peptide bonds. The amino acids present and the order in which they occur vary from one protein to another.
Protozoa are one-celled animals
A spore is a reproductive structure that is adapted for dispersal and surviving for extended periods of time in unfavourable conditions.
A poisonous or toxic substance - produced by pathogens.
A small amount of dead or weakened pathogen is introduced into the body. It prepares the immune system to prevent future infections with the live pathogen.
Medicine that contains a dead or weakened pathogen. It stimulates the immune system so that the vaccinated person has an immunity against that particular disease.
The smallest of living organisms. Viruses are made up of a ball of protein that contains a small amount of the virus DNA. They can only reproduce after they have infected a host cell.
Opportunistic Infections
An infection that would not normally cause disease in a healthy person but which can take hold when a person's normal immune defences are reduced.