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9.4 Search for better health: 2. Infectious and non-infectious diseases

Extract from Biology Stage 6 Syllabus (Amended October 2002). © Board of Studies, NSW

Background: Even though they did not know about microscopic disease-causing organisms, many social groups established rules and practices that protected people against infectious diseases. These practices resulted from observing cause and effect relationships. For example, the Chinese could have deduced the connection between water contaminated by faeces and gastro-intestinal diseases, and the Hebrews may well have made a connection between the symptoms of infection by tapeworms and eating undercooked pork.

distinguish between infectious and non-infectious disease

• An infectious disease is one that is caused by an organism and that can be transferred from one person to another. The transfer may be direct, where the disease-causing organisms, such as viruses or bacteria, pass directly from person to person, or it may be carried out by an intermediary (called a vector), such as a blood-sucking insect. Examples of infectious diseases are colds, influenza, chicken pox, herpes and measles.
• Non-infectious diseases are diseases that are not due to disease-causing organisms. They include genetic diseases, such as Down syndrome, haemophilia, and those that are related to lifestyle or environment, such as cardiovascular disease and skin cancer.

identify data sources, plan and choose equipment or resources to perform a first hand investigation to identify microbes in food or in water

• Agar is a jelly-like substance obtained from seaweed. When it is dissolved in water, nutrients suitable for microbes can be added to it before it sets to form a gel. If a microbe makes contact with the agar and if conditions are suitable, the microbe will grow and reproduce to form a visible colony.
• To design a first-hand investigation to identify microbes in food and water, firstly identify data about microbiological techniques from practical manuals or the Internet. Gather information about the techniques used to grow micro-organisms on agar plates in Petri dishes. You will need to know what sort of nutrients can be added to agar, the correct way of exposing the agar to the source of micro-organisms and the safety precautions that must be followed once a plate has been exposed. Also you will need to know the best temperature for microbial growth and how to distinguish between colonies of bacteria and fungi, growing on agar plates.
• Use the information to plan a valid and reliable investigation. You will need to decide on which nutrients you will add to the agar, what food and water samples you will test and how you will expose the agar to the microbes that may be present in your food and water samples. Decide on the temperature you will provide for microbial growth.
• Choose equipment and resources that will ensure your investigation is safe. Carry out a risk assessment of your experimental procedures and address potential hazards. In this type of investigation, the most important safety procedures are:
  • to seal and not open again the Petri dishes after exposure
  • to correctly autoclave exposed Petri dishes prior to final disposal.
• Perform your investigation. Dispose carefully and safely of the waste materials produced during the investigation. Safe working practices such as wearing latex gloves, washing benches and hands with chemical treatments for sterilisation are important when working with micro-organisms.
  Use data from the investigation to explain why cleanliness in food, water and personal hygiene practices are important.

explain why cleanliness in food, water and personal hygiene practices assist in the control of disease

• There are huge numbers of disease causing organisms. Most of them are microscopic and can enter the body through any body opening. The intake of food and water provide an easy way for micro-organisms to enter our bodies. Therefore, minimising the number of such organisms in our food and water reduces the risk of infection. Good personal hygiene ensures that body openings, including broken skin, are clean, so that the number of micro-organisms that might gain entry to our bodies is kept low. Since we cannot see individual micro-organisms we take precautions that we know will help to protect us. Examples of precautions include, washing hands after going to the toilet, cleaning wounds, boiling water and water treatment of water that doesn't come from a known safe source to make sure untreated sewage does not get into food or water supplies.

A new website that has some information on water quality is Water Quality Sydney Catchment Authority, 2008

identify the conditions under which an organism is described as a pathogen

• A pathogen is any organism that can produce a disease. Pathogens range from viruses so small that thousands will fit side by side in one millimetre to tapeworms that can be several metres long. They are all infectious.
• Pathogens may live outside the body, such as the fungus that causes ringworm, inside particular organs, such as parasitic worms in the intestine, in tissues or inside cells. Some information on water pathogens is found below.

  Water Pathogens Sydney Catchment Authority, 2008

gather, process and analyse information from secondary sources to describe ways in which drinking water can be treated and use available evidence to explain how these methods reduce the risk of infection from pathogens

Contamination of drinking water is a common way for pathogens to enter the body.

• Gather information from your local water supply authority or the Internet to find out the ways in which water can be treated and the range of pathogens that are targeted.

  Drinking water quality NSW Health

• Process your information by recording a description of each water treatment method and by looking for trends and patterns in why they are used.
  
  
  Information to get you started
  
  Examples of water treatments are filtration, chlorination and ozone filtration. In NSW water is filtered, chlorine is added to kill bacteria and samples are tested for the presence of coliform bacteria, giardia and cryptosporidium.
  

  Use questions as an effective processing technique to focus your attention when looking for trends in information sources. Some examples in this case might be “Which microbes are controlled by the treatment?”, “Which microbes are not controlled by the treatment?” and “What combination of treatments can be used to make a water supply safe for drinking?”

• Use the evidence you have gathered to propose logical explanations of how each form of water treatment protects people from the target pathogens.

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