Infectious Diseases - Pathogens

Page 6 of 10

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

Jump to the Page


Not all disease causing organisms are microorganisms. Many larger parasitic organisms also cause disease. A parasite is an organism that lives in or on another organism (the host). The parasite gets some or all of its food from the host organism. This can cause anything from mild disease to death to the host. Parasites can also spread diseases from one organism to another.

Some parasites feed on the outside or the host organism. Others live inside the tissues and cells of the body.

Parasitic organisms which affect the health of people include fleas and headlice which live on the outside of the body and tapeworms which live inside the body.

Parasites have adaptations which help them avoid being removed by the host organism. For example, fleas can jump great distances so they avoid being crushed. Their mouthparts can cut open the skin and suck blood. Big infestations can cause anaemia through lack of blood and let other infections in. Fleas carried the bacteria which caused the plague.

Headlice crawl about the head hidden in the hair. They stick their eggs firmly to the hair while they develop. They bite through the skin and suck the blood.

A flea

A flea
Photo courtesy of CDC/Janice Haney Carr

Tapeworms spend part of their lifecycle in an animal such as a pig or a cow, or part of their lifecycle in a human being. They have special hooks and sucker on their heads so they can attach themselves to the wall of the gut. They are very thin and can grow very long. They absorb digested food from the gut through their body wall. It is when they find their way into the tissue of the brain or the lungs that they can cause serious disease.

Tapeworm head

Tapeworms are well adapted to survive in the human gut
Photos provided by CDC

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.