Communicable Diseases


Health and Disease

- health is a state of physical and mental well- being

- diseases, both communicable and non-communicable are major causes of ill health

- other factors including diet, stress and life situations may have a profound effect on both mental and physical health

- different types of diseases may and often do interact

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Pathogens and Disease

- communicable diseases are caused by microorganisms called pathogens, which include bacteria, viruses, fungi and protists

- bacteria and viruses reproduce rapidly inside your body, bacteria can produce toxins that make you feel ill

- viruses live and reproduce inside your cells, causing cell damage

- pathogens can be spread by direct contact, by air or by water

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Growing Bacteria in the Lab

-an uncontaminated culture of microorganisms can be grown using sterilised Petri dishes and agar, you sterilise the inoculating loop before use and fix the lid of the Petri dish to prevent unwanted microorganisms getting in

- uncontaminated cultures of microorganisms are needed for investigating the action of disinfectants and antibiotics

- cultures should be incubated at a maximum temperature of 25°C in schools and colleges to reduce the likelihood of pathogens growing that might be harmful to humans

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Preventing Bacterial Growth

- bacteria multiply by simple cell division as often as every 20 minutes if they have enough nutrients and a suitable temperature

- can investigate the effects of disinfectants and antibiotics on bacterial growth using agar plates and calcuating the cross- sectional area of colonies grown or of clear areas of agar

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Preventing Infections

- the spread of disease can be prevented by simple hygiene measures,

    by destroying vectors

    by isolating infected individuals


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Viral Diseases

- measles virus is spread by droplet infection, it causes fever and a rash, can be fatal, there is no cure, isolation of patients and vaccination prevents spread

- HIV initially causes flu-like illness, unless it is successfully controlled with antiretroviral drugs the virus attacks the bodys immune cells

    late stages HIV infection or AIDS occurs when the bodys immune system becomes so badly damaged it can no longer deal with other infections or cancer

    HIV is spread by sexual contact or by the exchange of body fluids, such as blood, which occurs when drug users share needles

- tobacco mosaic virus is spread by contact and vectors, it damges leaves and reduces photosynthesis, there is no treatment, its spread is prevented by field  hygiene and pest control

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Bacterial Diseases

- salmonella is spread through undercooked food and poor hygiene

    symptons include fever, abdominal cramps, diarrhoea and vomitting caused by the toxins produced by the bacteria

    in the Uk, poultry are vaccinated against salmonella to control the spread of disease

- gonorrhoea is a sexually transmitted disease

    symptons include discharge from the penis and vagina and pain on urination

    treatment involves using antibiotics, although many strains are now resistant, using condoms and limiting sexual partners prevents spread

- there are relatively few bacterial diseases of plants but Agrobacterium tumefaciens causes galls

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Diseases Caused by Fungi and Protists

- rose black spot is a fungal disease spread in the environment by wind and water, it damages leaves so they drop off, affecting growth as photosynthesis is reduced

    spread is controlled by removing affected leaves and chemical sprays, but is not very effective

- malaria is caused by parasitic protists and is spread by the bite of female mosquitos, it damages blood and liver cells, causes fevers and shaking, and can be fatal

    some drugs are effective if given early but protists are becoming resistant, spread is reduced by preventing the vectors from breeding amd by using mosquito nets to prevent people from being bitten

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Human Defence Responses

- your body has several lines of defence against the entry of pathogens, these include the skin (barrier and antimicrobial), the nose, the trachea, the bronchi and the stomach

- white blood cells help to defend against pathogens by ingesting them and making antibodies and antitoxins 

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More About Plant Diseases

- plants can be infected by a range of viral, bacterial and fungal pathogens as well as insect plants 

- plants can be damaged by a range of ion defciency conditions

- plant diseases can be detected by a range of symptons and identified in a number of ways, including gardening manuals and laboratory tests, some involving monclonal antibdodies

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Plant Defence Responses

- physical plant defences against invasion by microorganisms include cellulose cell walls, tough waxy cuticles and layers of bark or dead cells (or dead leaves) which fall off

- chemcial plant defences include antibacterial chemicals and poisons to deter herbivores

- many plants have mechanical adaptions against herbivores such as thorns and hairs, leaves that droop or curl when touched and mimicry to trick animals

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