Biology - Infection and disease Notes

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Biology - Infection and Disease 11.4

Bacteria, Fungi and Viruses are the three types of micro-organisms that cause disease. They reproduce rapidly because: 

  • humans are warm (37 degrees C)
  • humans are moist
  • humans have sugars on the surface of our cells


Pathogens are microorganisms that cause infectious disease. Bacteria and viruses are the main pathogens. It enters the body through gaps in the skin. 

 Bacteria and Viruses

Bacteria are microscopic single-celled organisms. They come in many shapes and sizes, but even the largest are only 10 micrometres long - 10 millionths of a metre. Bacteria are living cells and, in favourable conditions, can multiply rapidly. Once inside the body, they release poisons or toxins that make us feel ill. Diseases caused by bacteria include:

  • food poisoning
  • cholera
  • typhoid
  • whooping cough
  • gonorrhoea - a sexually transmitted disease 

Viruses are many times smaller than bacteria. They are among the smallest organisms known and consist of a fragment of genetic material inside a protective protein coat. Viruses can only reproduce inside host cells, and they damage the cell when they do this. A virus can get inside a cell and, once there, take over and make hundreds of thousands of copies of itself. Eventually the virus copies fill the whole host cell and burst it open. The viruses are then passed out in the bloodstream, the airways, or by other routes.  Diseases caused by viruses include: 

  • influenza - flu
  • colds
  • measles
  • mumps
  • rubella
  • chicken pox
  • AIDS


The body has different ways of protecting itself against pathogens. The first defence is passive immunity. This is aimed at stopping the pathogen getting into the body in the first place. The body’s passive immunity system includes the skin, mucus and cilia in the respiratory system, acid in the stomach, and enzymes in tears. If a pathogen still manages to get into the body, the second defence takes over. This is called active immunity, and the white blood cells have key functions in this.

White blood cells  

A white blood cell ingesting disease-causing bacteria.
White blood cells can: 

  • ingest pathogens and destroy them
  • produce antibodies (things which attach themselves to the antigens of the pathogen)  to destroy pathogens
  • produce antitoxins that neutralise the toxins released by pathogens

also a point that... 

  • pathogens are not the disease – they cause the disease
  • white blood cells do not eat the pathogens – they ingest them
  • antibodies and antitoxins are not living things – they are specialised proteins

More about white blood cells 

There are several different types of white blood cells, each with different functions, but they can



A detailed set of revision notes on microbes and disease that could be used by any student studying this topic. Use these to make your own set of flashcards for all the important key words with their definitions for an extra resource.

Adam Shabana

hahah Yeah NO


Very useful!


helped me with my GCSE's!

Bananaexcel 2.0

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