Infectious Diseases - Pathogens

Page 8 of 10

  • 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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How pathogens cause disease


Once you are infected bacteria grow and reproduce inside the cells of your body. As they grow and divide, many bacteria make toxins (poisons). These toxins can damage your cells. Some bacteria damage your cells directly as they grow. Disease symptoms such as a high temperature, headaches and rashes can be caused by the damage and toxins or by the way your body responds to the damage and toxins produced by the bacteria.


Viruses cannot reproduce outside of the cells of their host. Once you are infected, viruses take over the genetic material of your cells. In this way they direct your cells to make copies of the virus until eventually the cell bursts, releasing many more viruses to infect other cells. In this way viruses kill infected cells, spread and damage body tissues. The way your body responds to the bursting of the cells causes the symptoms of the disease.

Animation showing how a virus infects a cell.

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Preventing infection

As you have seen, there are pathogens all round us and many different ways they can be spread from one person to another. Your body needs to be able to stop these pathogens from gaining entry. This part of the immune system is called non-specific immunity because it is present all the time and not activated in response to a particular pathogen.

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So you think you are clean?

Part of body Bacteria
Head (scalp) 1,000,000 /cm2
Surface of skin 1000 /cm2
Saliva 100,000,000 /g
Nose mucus ('snot') 10,000,000 /g
Faeces over 100,000,000 /g

The table shows the numbers of bacteria that are living on you and inside you all the time. It is easy to see why your body needs barriers to prevent pathogens entering and causing a serious infection.


Your skin is possibly the most important barrier to prevent infection entering the body. Cuts and grazes can break this barrier and there are systems to automatically repair any damage. Your skin is a specialised organ that not only protects but also senses the environment and helps to regulate your body temperature.

Breaking the barrier

The outer layer of skin forms a tough barrier to infection that is effective as long as it is intact. Cuts, grazes, burns and hypodermic syringes are all ways that this barrier can be broken and pathogens gain entry into the body. To prevent this happening, the skin can repair itself when any damage occurs.

Click on the numbers below to see the steps of repair when the skin is cut.

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  • Stage 1
    a sharp object cuts the skin.
  • Stage 2
    blood enters the wound. A blood clot quickly forms to seal the cut and prevent blood loss and entry of pathogens. The clot dries to form a scab.
  • Stage 3
    new skin grows to repair the damage caused by the cut. After a week or two the scab falls off.
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.