HSC Biology Module 7: Infectious Disease

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Module 7: Infectious Disease


Causes of Infectious Disease

Describe a variety of infectious diseases caused by pathogens, including microorganisms, macroorganisms and non-cellular pathogens, and collect primary and secondary-sourced data and information relating to disease transmission, including:

– classifying different pathogens that cause disease in plants and animals

Pathogens can either be cellular or non-cellular:

Cellular pathogens Non-cellular pathogens
Bacteria: prokaryotes Virus
Fungi: eukaryotes e.g. yeast Viroids (lack protein coat)
Oomycetes: protists Prions (forms faulty proteins)
Protozoa: eukaryotes
Endoparasites: macroscopic organisms that live inside of organism
Ectoparasites: macroscopic organisms that live outside of organism


Pathogen Form of transfer to plants Form of transfer to animals
Bacteria Soil, water, animals and injuries to animals caused by agricultural tools e.g. grafting or propagation



Fungi COMMON DISEASE: POWDERY MILDEW (forms white powder spots and blackens leaves → reduces photosynthesis) COMMON DISEASE: THRUSH
Oomycetes Spores are released onto a leaf and carried in water droplets to other leaves


COMMON PATHOGEN: Phytophthora cinnamomi

Exposure to stagnant water that contains spores of oomycetes


COMMON PATHOGEN: Pythium insidiosum

Protozoa COMMON DISEASE: DOWNY MILDEW Water with contaminated faeces


COMMON PATHOGEN: Giardia lamblia (infects small intestines → diarrhoea)

Endoparasites COMMON PATHOGEN: ROUNDWORMS (HELMINTH) → infect plant roots COMMON PATHOGEN: TAPEWORM (HELMNITH) → prevent nutrient absorption on animals
Virus Vectors (insects) sucking on diseased plants and piercing healthy plants




– investigating the transmission of a disease during an epidemic

Epidemic Pandemic
Large occurrence of a specific infection across a population. Large occurrence of a specific infection across multiple countries.

Epidemics occur because of:

  • Increased virulence of pathogen
  • Increased migration of pathogen
  • More host exposure/susceptibility to pathogen


  • Ebola (originates in bat meat) occurred because of increased virulence of pathogen as human consumption of bat meat increased.
  • Spanish flu occurred because of increased migration of pathogen as people travelled to different countries and introduced the disease to new populations.
  • Cholera occurred because of increased host exposure/susceptibility as the waterways became contaminated and were simultaneously used as drinking water.

– design and conduct a practical investigation relating to the microbial testing of water or food samples


  1. Clean work area with ethanol → no contamination.
  2. Collect food and water samples in individual test tubes and stopper them. Food: mash up and put in a test tube with 2mL of distilled water.
  3. To sterilise inoculating loop move through blue flame of a Bunsen burner until it glows. Allow to cool.
  4. Remove stoppers from test tubes and sterilise mouth of test tubes by moving through blue flame of Bunsen burner.
  5. Open the petri dish: the dish should be on the table and the lid lifted at an angle no greater than 45° with opening away from people.
  6. Use the inoculating loop to spread sample in zig-zag motion onto nutrient agar plate and immediately seal plate with sticky tape.
  7. Repeat steps 3-6 with other samples.
  8. Seal one plate with no sample and label ‘control’.
  9. Incubate plates upside down at 25° for 7 days.
  10. Record observations of colony type, form, elevation, margin and colour.

SAMPLE RISK ASSESSMENT: Hands should be washed thoroughly to remove any microbes from the skin which could cause disease.

– investigate modes of transmission of infectious diseases, including direct contact, indirect contact and vector transmission

Direct: transmission of pathogens from infected individual to healthy individual via direct contact.

Indirect: transmission of pathogens with no direct contact.

Vector: transmission of pathogens from infected individual to healthy individual via another animal e.g. mosquitos in malaria.

Investigate the work of Robert Koch and Louis Pasteur, to explain the causes and transmission of infectious diseases, including:

– Koch’s postulates – Pasteur’s experiments on microbial contamination

  • Scientists disproved the spontaneous generation theory and replaced it with the germ theory


  • Discovered 1 pathogen = 1 disease.
  • Discovered anthrax bacilli caused anthrax disease in sheep: Isolated rod shaped bacteria and grew in cultures. He injected this into sheep and repeatedly showed that it caused anthrax disease.
  • Identified bacteria responsible for cholera.
  • Technology:

– Discovered agar plate for growing micro-organisms. Used this to culture anthrax bacillus.

– Microscope: see micro-organisms and their features.

– Photography: photograph micro-organisms.

– Staining techniques with dyes: see bacteria and specific features.

Koch’s postulates:

  1. Same micro-organism must be present in every diseased host.
  2. Micro-organism isolated and grown in pure culture.
  3. Healthy host infected with pure culture must produce same symptoms as original host.
  4. Micro-organism re-isolated from 2nd host, grown in pure culture and identified as original.
  • Disproved spontaneous generation theory with swan necked flask experiment: curved neck had no microbial growth, straight neck had microbial growth, furthermore if curved neck flask was tipped to allow solution to reach curve there was growth.

  • Discovered micro-organisms in AIR cause disease.
  • Established germ theory of disease: germs cause disease and all micro-organisms come from pre-existing micro-organisms.
  • Developed pasteurisation (destroy microbes using heat).
  • Discovered fermentation was due to yeast.
  • Discovered anthrax was due to spores from carcasses being grazed by healthy animals.
  • Established principle of immunity: Discovered way to weaken microbes = immunisation = vaccines for chicken cholera and anthrax.

COLLABORATION: Koch discovered anthrax pathogen which Pasteur developed a vaccine for.

Assess the causes and effects of diseases on agricultural production, including but not limited to:

– plant diseases


  • Lack of genetic variation.


  • Lower yield and quality of crops.

Example: potato blight

  • Potatoes grown asexually → genetically identical potatoes → fungal pathogen introduced to population → every potato is susceptible to disease → Irish potato famine

– animal diseases


  • Intensive (increased) farming with higher volumes of animals → increased contact between animals → easier disease transmission.
  • Increased human consumption of meat, eggs and dairy → easier disease transmission.


  • Change in farming practices to prevent transmission of future disease e.g. vaccines and antibiotics.
  • Increased biosecurity and border control to prevent introduction of new diseases.

Example: mad cow disease

  • Cows carry infectious prion prion → human consumption of cow meat → Creutzfeldt-Jakob disease.


Compare the adaptations of different pathogens that facilitate their entry into and transmission between hosts


  • Adaptation: Parasite developed a mutation which made it resistant to chloroquine (antimalarial drug).
  • Adaptation: Uses a host (mosquitos) to penetrate skin (physical barrier) and transmit disease.


  • Adaptation: Uses a host (fleas).


  • Adaptation: Uses chemotaxis to move through mucus (barrier) and transmit disease.
  • Adaptation: Can change to pH of surrounding environment to survive in organism.


  • Adaptation: Constantly produces new antigens that organisms don’t recognise.


  • Adaptation: Destroys specialised white blood cells that protect the organism.


Responses to Pathogens

Investigate the response of a named Australian plant to a named pathogen through practical and/or secondary-sourced investigation, for example:

– fungal pathogens – viral pathogens

Oidium eucalypti (fungal) → causes powdery mildew disease in Eucalyptus.


  • Formation of barrier zones in new tissue → protect new healthy tissue from nearby infected tissue.
  • Induced responses → plant identifies infected cells and induces cell death.

Analyse responses to the presence of pathogens by assessing the physical and chemical changes that occur in the host animals cells and tissues


Barrier Description + example of disease if barrier is penetrated Assessment
Skin Outer body protection

Disease: ringworm

Positives: difficult to penetrate.

Negatives: once penetrated, the organism is infected immediately.

Cilia Hair-like structrues that sweep pathogens from body

Disease: sinus infections

Positives: found in many body structures to prevent entry of pathogens.

Negatives: pathogens easily get past cilia.

Peristalsis Muscle contractions to help move mucus or kill pathogens (usually precursor to vomiting)

Disease: esophagitis

Positives: kills pathogens in body.



Barrier Description + example of disease if barrier is penetrated Assessment
Mucous membranes Secrete viscous substance to trap pathogens

Disease: cholera

Positives: prevents entry of pathogens.


Sebum Makes skin slightly acidic which disfavours survival of pathogens

Disease: folliculitis

Positives: kills pathogens in body.

Negatives: some pathogens have adapted to change surrounding pH, enabling survival.

Cerumen (ear wax) Traps pathogens in ear canal

Disease: ear infection

Positives: prevents entry of pathogens.

Negatives: build up of ear wax causes blockage of eardrum leading to other infections.

Lacrimal secretions (tears) Tears contain lysozymes that can destroy pathogens

Disease: conjunctivitis

Positives: removes AND protects from pathogens.
Acidic secretions Acidic secretions (low pH) kill pathogens. Positives: kills pathogens in body.

Negatives: secretions cannot be too acidic, otherwise the organism will be harmed.

Lysozymes (found in tears and saliva) Destroy bacterial cell walls. Positives: kills pathogens in body.

Negatives: cannot kill pathogens without cell walls.


  • Inflammation: increases leakiness of blood vessels for efficient travel of white blood cells and signals phagocytes to travel to site of infection.
  • Fever: increased body temperature can slow down pathogen proliferation and increase white blood cell proliferation.


Investigate and model the innate and adaptive immune systems in the human body


  • Non-specific
  • Rapid
  • Fixed responses (non-specialised, same for all pathogens)
  • No immunological memory


  • Phagocytes (neutrophils, macrophages, monocytes, dendritic cells)


  • Specific
  • Lag phase for first exposure to antigen
  • Immunological memory (no lag phase for second exposure to antigen)


  • Helper T lymphocytes
  • Cytotoxic T lymphocytes
  • Suppressor T lymphocytes (stop immune response)
  • Memory T lymphocytes (remember specific antigen to reduce lag phase upon second exposure)
  • Plasma B cells (secrete antibodies)
  • Memory B cells (remember specific antigen to reduce lag phase upon second exposure)

Explain how the immune system responds after primary exposure to a pathogen, including innate and acquired immunity

Primary exposure: first line of defence was unsuccessful.




  • Neutralise toxins and pathogens.
  • Aggluntination: activation of phagocytes.
  • Precipitation: antibodies bind to antigen to become insoluble and precipitate out of solution.


  • Site for lymphocytes to mature and survive.
  • Transports lymphocytes and antigen-presenting cells to lymph nodes to activate the adaptive immune response.
  • Removes debris from circulation.

Prevention, Treatment and Control

Investigate and analyse the wide range of interrelated factors involved in limiting local, regional and global spread of a named infectious disease

HENDRA VIRUS (bats transmitted disease to horses to humans)

  • Horse owners were educated on the common symptoms (fever, breathing difficulties) → seek early treatment.
  • Vaccine was invented for immunising horses → prevention.


Investigate procedures that can be employed to prevent the spread of disease, including but not limited to:

– hygiene practices

Personal hygiene

  • Wash hands, teeth, body and clothes.
  • Clean house.
  • Cough/sneeze into your own hand.
  • Protected sex.
  • Avoid sharing food/drink.

Government regulations

  • Provides clean water.
  • Correct disposal of sewerage and waste.

Safe food practices

  • Wash hands before handling food.
  • Clean food utensils and benches.
  • Store food correctly and do not cross contaminate.
  • Do not cough/sneeze onto food.

– quarantine

Quarantine: an isolated place where people or transported items are placed to prevent exposure to an infectious disease e.g.

  • Isolating patients in a hospital to contain an infectious disease.
  • Isolating transported animals/produce and physically examining them to prevent spread/introduction of infectious disease.

– vaccination, including passive and active immunity

  • Protects individuals from disease.
  • Builds herd immunity (group immunity).
Passive immunity Active immunity
Artificial (vaccination) –       Injection of antiserum (antibodies from another organism). –       Antigens are introduced to induce adaptive immune response.

–       First-time exposure to antigen will be strong and fast.

Natural –       Occurs during pregnancy.

–       Mother transfers antibodies to fetus. Antibodies can attack antigens.

–       Develops from the adaptive immune response.

–       First-time exposure to antigen will have a lag phase.



– public health campaigns

Educate and warn individuals on transmission and prevention of infectious disease e.g. when Sydney water was infected with Cryptosporidium and Giardia a campaign was launched to:

  • Educate public about cause and transmission of disease.
  • Stop people drinking unbottled water.
  • Stop people visiting pools.
  • Inform people how to boil water.
  • Inform people of common symptoms and treatments.

– use of pesticides

Pesticides: kill plant/animal pathogens and kill insect vectors e.g. mosquitos for malaria.

Disadvantage of using pesticides: animals/plants develop genetic resistance via natural selection → pesticides become ineffective.

– genetic engineering

  • Produce plants/animals that are disease resistant.
  • Engineer animals/plants that can produce antibodies for vaccines.
  • Produce vectors that cannot spread disease as efficiently.

EXAMPLE: COWS are being engineered to efficiently produce lots of antibodies for vaccines.

Investigate and assess the effectiveness of pharmaceuticals as treatment strategies for the control of infectious disease, for example:

– antivirals


  • Viruses have a protein coat (capsid) and some have envelope proteins which form when the virus replicates. Viruses use host’s ribosomes for protein synthesis. Antivirals target capsids, envelope proteins or polymerase enzymes (to prevent protein synthesis).


  • Pathogens commonly develop genetic resistance via natural selection rendering the antiviral ineffective.
  • Does not work on a wide range of pathogens (only viruses).

– antibiotics


  • Bacteria can be killed with antibiotics e.g. penicillin.
  • Effectiveness of antibiotics can be improved through chemical modifications.
  • Antibiotics can break down bacterial cell wall OR inhibit bacterial metabolic pathways OR inhibit bacterial transcription of mRNA to stop protein synthesis.


  • Works against SOME pathogens e.g. doesn’t kill viruses.
  • Most antibiotics are naturally occurring from fungi/bacteria → large amounts of antibiotics can be efficiently produced.
  • Pathogens commonly develop genetic resistance via natural selection rendering the antibiotic ineffective.

Investigate and evaluate environmental management and quarantine methods used to control an epidemic or pandemic

Disease: Whooping cough (endemic) caused by bacteria Bordetella pertussis

Management methods

  • Encourage vaccination of the population (especially families with babies) to develop herd immunity → babies will not get infected despite not being vaccinated.
  • Limit food preparation from infected individuals.
  • Prevent infected individuals sharing food.

Quarantine methods

  • Isolate infected patients in hospitals.
  • Isolate infected individuals in homes.
  • Prevent infected patients from travelling.

Interpret data relating to the incidence and prevalence of infectious disease in populations, for example:

Incidence: rate of new cases developing for a particular disease.

Prevalence: percentage of cases in a population at a given time.

– mobility of individuals and the portion that are immune or immunised


  • Increased travel around the world (accessibility to air travel) → increases exposure across the world → increases incidence.


  • Prevalence depends on implementation of health campaigns, vaccinations and education about sanitation.

– Malaria or Dengue Fever in South East Asia


  • Vaccinations were invented and administered → decreased incidence.
  • Global health campaigns and warnings about travelling to countries infected with malaria was discouraged → decreased incidence.


  • Pesticides against mosquitos were developed → decreased prevalence.
  • Education on sanitation and cause/symptoms of malaria → decreased prevalence.

Evaluate historical, culturally diverse and current strategies to predict and control the spread of disease


  • Epidemiology (current and historical): study quantitative patterns of disease → helps discover cause and common symptoms of disease.
  • Disease is caused by supernatural forces and spiritual help must be sought (cultural).


  • Seeking supernatural help (cultural).
  • Vaccination and health campaigns (current).
  • Sanitation (current and historical).

Investigate the contemporary application of Aboriginal protocols in the development of particular medicines and biological materials in Australia and how recognition and protection of Indigenous cultural and intellectual property is important, for example:

– bush medicine

  • Plants are used for healing.
Plant Application
Kangaroo apple Heals swollen, inflamed joints.
Sticky hopbush Heals ear aches.
Lemongrass Helps treat fevers.

– smoke bush in Western Australia

  • Smoke bush contains essential oils which have medicinal benefits.
  • Currently being used in cancer research and to treat HIV. Smoke bush contains Conocurovone which has potential to kill viruses.


  • Australia aims to implement an agreement with Indigenous people.
  • Agreement: enables use of native plants/animals/traditions for medicinal purposes with the acknowledgment of Indigenous people.
  • Outcome: Indigenous people will be justly acknowledged and receive a percentage of financial revenue.
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