what is a disease
disease is not simple thing, but rather a description of certain symptoms either physical or mental, or both. disease suggests a malfunction of body or mind which has an aderse effect on good health.
micro-organisms as pathogens
-gain entry to a host
-colonise the tissue of the host
-resist the defences of the host
-cause damage to the host tissues
pathogens include bacteria, virus and fungi.
if a pathogen gets into a host and colonises its tissue an infection results. disease occurs when an infection leads to recognisable symptoms in the host. when a pathogen is transferred from one individual to another it is known as transmission.
how do micro-organisms get into the body
the gas-exchange system: many pathogens enter the body through the gas-exchange surfaces. pathogens that cause influenza, tuberculosis and bronchitis infect this way.
the digestive system: food and water may carry pathogens into the stomach and intestines via the mouth. cholera, typhoid and dysentery pathogens enter the body by this means.
- a mucuous layer that covers exchange surfaces and orms a thick sticky barrier that is difficult to penetrate
- the production of enzymes that break down a pathogen
- the production of stomach acid, which kills micro-organisms
how do pathogens cause disease?
by damaging host tissues:sometimes the sheer number of pathogens causes damage, by e.g. preventing tissues functioning properly. viruses inhibit the synthesis of dna, rna and proteins by the host cells. many pathogens break down the membranes of the host cells.
by producing toxins:most bacterial pathogens produce toxins. the cholera bacterium produces a toxin that leads to excessive water loss from the lining of the intestines.