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Pathogens and the immune system

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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 sprouting from a chronically infected cultured animal cell.

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.