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Science

Disease and resistance

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The body’s first line of defence against harmful pathogens is the skin and stomach acid, the second is white blood cells. Vaccination involves exposing the body’s immune system to a weakened or harmless version of the pathogen in order to stimulate white blood cells to produce antibodies.

Pathogens

Pathogens are microorganisms that cause infectious disease. Bacteria and viruses are the main pathogens.

Bacteria

the cell is oval, with tail-like features attached to the outside

Salmonella bacterium cell

Bacteria come in many shapes and sizes, but even the largest are only ten micrometres long (ten-millionths of a metre).

Bacteria are living cells [cell: Basic unit of life. Unicellular organisms only have one cell. Multicellular organisms have many cells. ] and can multiply rapidly in favourable conditions. Once inside the body, they release poisons or toxins [toxins: a type of natural poison produced by an organism, often as a form of protection ] that can make us feel ill.

Viruses

spherical shaped virus showing a cross-section through the core

A hepatitis C virus showing DNA enclosed in a protein coat.

Viruses are many times smaller than bacteria. They are among the smallest organisms [organism: A living being - plant, animal, fungus or bacterium. ] known, consisting of a fragment of genetic material inside a protective protein coat.

Viruses can only reproduce inside host cells, damaging them when they do so. Once inside, they take over the cell and make hundreds of thousands of copies of themselves. Eventually, the virus copies fill the whole host cell and it bursts open. The viruses then pass out through the bloodstream, the airways, or by other routes.

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