Disease Notes

Edexcel GCSE Biology: Revision notes

Key facts

  • Health is complete physical, mental and social well-being, not just the absence of disease.
  • Communicable diseases are caused by pathogens; non-communicable diseases cannot be transmitted.
  • Know seven named diseases: cholera, TB, stomach ulcers, Chalara ash dieback, malaria, HIV and Ebola.
  • Viruses replicate by the lytic pathway (burst) or the lysogenic pathway (dormant).
  • Plants have physical and chemical defences, and diseases are detected by symptoms and diagnostic tests.

Communicable

  • Caused by a pathogen
  • Spreads between organisms

Non-communicable

  • Cannot be transmitted

Health and disease

The WHO defines health as complete physical, mental and social well-being, not just the absence of disease.

One disease can increase susceptibility to others: HIV, for example, weakens immunity to other infections. Diseases are either communicable or not.

  1. 1

    HIV infection

    a communicable disease caused by a virus

  2. 2

    Immune system weakened

    fewer white blood cells work properly

  3. 3

    Other infections

    become more likely and more serious

One disease can make another more likely: the example of HIV.

Communicable

  • Caused by a pathogen
  • Spreads between organisms

Non-communicable

  • Cannot be transmitted

Which is a communicable disease?

Pathogens and named diseases

Know seven named diseases, with the type of pathogen and the effect of each.

A pathogen is a disease-causing organism. Bacteria, fungi, protists and viruses are all pathogens. HIV destroys white blood cells and can lead to AIDS.

Bacteria

  • Cholera: diarrhoea
  • TB: lung damage
  • Stomach ulcers (Helicobacter): stomach lining damage

Fungi and protists

  • Chalara ash dieback (fungi): leaf loss and bark lesions
  • Malaria (protists): blood and liver damage

Viruses

  • HIV: destroys white blood cells
  • Ebola: haemorrhagic fever
Cell wallCell membranePlasmidCytoplasmChromosomal DNAFlagellumRibosome
A bacterial cell, a type of pathogen

Which pathogen type causes malaria?

Spread and prevention

Prevention targets the specific route of spread: clean water, vaccination, nets, hygiene and isolation.

Malaria is spread by a vector, the mosquito, so controlling mosquitoes with nets reduces it. Chlamydia (bacteria) and HIV (virus) spread by sexual contact, so condoms and screening reduce them.

  1. 1

    Mosquito bites an infected person

    picks up the pathogen

  2. 2

    Mosquito bites another person

    passes the pathogen on

  3. 3

    Malaria develops

    in the newly infected person

  4. 4

    Prevention

    nets and mosquito control break the chain

Malaria spreads through a vector, which is where prevention can break the chain.

Water and oral

  • Cholera: water
  • Stomach ulcers: oral

Airborne

  • TB
  • Chalara ash dieback

Vectors

  • Malaria: mosquitoes

Body fluids

  • Ebola
  • HIV and chlamydia: sexual contact

How can the spread of malaria be reduced?

The viral lifecycle

Viruses replicate by the lytic pathway, which bursts the host cell, or the lysogenic pathway, which stays dormant.

In the lytic pathway the virus injects its genetic material, the host makes new viruses and the host cell bursts. In the lysogenic pathway viral DNA inserts into the host chromosome and is copied as the cell divides. It stays dormant but may later switch to the lytic pathway.

Surface proteinsProtein coatGenetic materialEnvelope
A virus particle
  1. 1

    Inject

    virus injects its genetic material

  2. 2

    Replicate

    host cell makes new viruses

  3. 3

    Lysis

    host cell bursts, releasing the viruses

Lytic pathway
  1. 1

    Insert

    viral DNA joins the host chromosome

  2. 2

    Dormant

    copied each time the cell divides

  3. 3

    Switch

    may later enter the lytic pathway

Lysogenic pathway

In which pathway does the viral DNA stay dormant?

Plant defences and detection

Plants have physical and chemical defences, and disease is detected by ruling out causes and testing.

Physical defences are the cell wall and cuticle. Chemical defences include antibacterial compounds, some of which are used in human medicine: willow bark contains a compound related to aspirin.

  1. 1

    Observe symptoms

    for example spots, wilting, discolouration

  2. 2

    Eliminate environmental causes

    such as lack of minerals or water

  3. 3

    Distribution analysis

    where in the field or region it appears

  4. 4

    Diagnostic testing

    identify the pathogen

Detecting plant disease.

Physical

  • Cell wall
  • Cuticle

Chemical

  • Antibacterial compounds
  • Some used in medicine

Detection

  • Eliminate environmental causes
  • Distribution analysis
  • Observe symptoms
  • Diagnostic testing

Name two physical defences of plants.

Try an exam question

Describe the lytic and lysogenic pathways of virus replication.

[4 marks]

That's the notes covered.

Carry on to the next subtopic.