Causes of disease

Pathogens: Micro-organisms that cause disease

What is disease?  Malfunction of body or mind that has an adverse effect on good health

For MO to be pathogen, it must: gain entry to host colonise tissues of host resist defences of host cause damage to host tissues

Infection: When pathogen enters host, colonises its tissue

Disease: When infection leads to recognisable symptoms

Transmission: Pathogen transferred from one individual to another

How Micro organisms get into the body? By penetrating one of organism's interfaces with environment Interface: links external environment with internal environment of body Invasion occurs only when skin is broken

Gas-exchange system: Pathogens enter body through gas exchange surfaces Pathogens that cause influenza, tuberculosis and bronchitis infect this way

Digestive system: Food and water may carry pathogens into stomach, intestines via mouth Cholera, typhoid and dysentery pathogens enter body this way

Natural defences to prevent entry of pathogens: Mucous layer that covers exchange surfaces and forms a thick sticky barrier that is difficult to penetrate Production of enzymes that break down the pathogens Production of stomach acid, which kills micro organisms

How do pathogens cause disease? By damaging host tissues: Prevent tissues functioning properly, Viruses inhibit the synthesis of DNA, RNA and proteins by host cells By producing toxins: Bacterial pathogens produce toxins, Cholera bacterium produces toxin that leads to excessive water loss from lining of intestines

Extent of damage: How quickly a pathogen causes damage and hence onset of symptoms related to how rapidly pathogen divides

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