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Communicable disease

Diseases that can be transmitted from one individual to another.

Immune system

The body's natural defence mechanism against infectious diseases.


A bacterium, virus, or other microorganism, capable of causing disease.


A list of often difficult or specialised words with their definitions.


A form of asexual reproduction in which a new individual is produced as an outgrowth (bud) of the parent and is later released as a clone of the parent.


An agent that carries a pathogen from one organism to another. In genetics, a vector is a virus used to transfer genetic material into a cell


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


The basic unit from which all living organisms are built up, consisting of a cell membrane surrounding cytoplasm and a nucleus.

Pathogens and the immune system

Communicable or infectious diseases cause millions of deaths globally every year. These are diseases that are caused by pathogens. The human immune system has evolved to protect us from infection as far as possible. Understanding how pathogens cause disease, and how a healthy body protects us against pathogens, can help us reduce the impact of communicable diseases around the world

Virus are so small that we need an electron microscope to see them in any detail – but they can cause disease and death. The blue Ebola virus particles in this image are budding from a chronically infected cultured animal cell.

SEM of Ebola virus

Image courtesy of National Institute of Allergy and Infectious Diseases (NIAID)

Many different organisms can cause human disease. We are under attack from many organisms, from tiny, geometric viruses to parasitic worms that can grow many metres long inside our bodies. The symptoms of disease are a result of the way our bodies respond to the invasion of these foreign organisms.

Pathogens spread from one host to another in a variety of ways. Every time you touch a surface, you deposit microorganisms including pathogens from the surface of your skin. At the same time you collect microorganisms from everyone else who has touched that surface, or coughed or sneezed nearby. Other people can be vectors of disease, and so can other animals. From mammals such as dogs and bats carrying rabies, to mosquitoes carrying malaria, yellow fever and dengue and house flies which can carry around 100 different pathogens into our kitchens, the world we inhabit is full of disease-causing organisms. So why are so many of us free from infections most of the time?

Our bodies have an amazing range of defences against the invasion of pathogens. Some of these are very general; others are specific to individual types of pathogen. The more we understand about the complex interactions between our bodies and the organisms that cause disease, the more chance we have of reducing the immense toll of human suffering and loss of life that results from these infections every year.

Animations and diagrams by Edward Fullick throughout. Photos by Anthony Short unless credited otherwise.

How to use this site

There are a number of interactive features in this e-source:

  • A glossary of terms: any word with a glossary entry is highlighted like this. Moving the mouse over the highlighted word will show a definition of that word.
  • Quick questions: at the end of most pages or sections there is a question or set of quick questions to test your understanding.
  • Animations: most of the animations can be expanded to full screen size, ideal for showing on an interactive whiteboard. The animations will play all the way through or can be viewed one section at a time.
  • Downloads: Teachers can download individual diagrams, animations and other content from the Download Library area of the website. Terms and Conditions apply.