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Infectious disease

Infectious diseases are caused by the transmission of pathogens, which are microorganisms such as bacteria and viruses. Pathogens can be transmitted by direct contact - horizontal and vertical transmission - and by indirect contact - vehicle-borne transmission and vector-borne transmission.


Pathogens are microorganisms that cause infectious disease. Pathogens are mostly bacteria but some are viruses, fungi and protoctists.


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 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.


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 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 of the body in the bloodstream, the airways, or by other routes.

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