Communicable diseases


Key words

Health- Free from disease / illness - Physical and mental and social wellbeing -Good nutrition-Suitably housed

Disease- A departure from good health caused by a malfunction of the mind or body  

Communicable Disease - diseases which can be passed from one organism to another of the same or different species

Parasite- lives on or in a host organism and harms host - takes nutrition from the host or feeds on the host

Pathogen- Microorganism which causes disease

Benefit of parasite living on host- warmth- protection / safe place- allows transmission / spread to a new host

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Bacteria- Prokaryotic- single celled organism

  • reproduce rapidly by binary fission (mitosis)
  • cause harm by releasing toxins/toxic waste products

Fungi- eukaryotic organisms

  • release digestive enzymes into surrounding tissue

Protoctista- eukaryotic organisms- Single celled

  • enter host cells and feed on contents (parasitic)


  • can only multiply inside host cells
  • viral  DNA inserted into host DNA
  • host cell transcribes and translates viral proteins and replicates viral DNA – creating new viral particles
  • cell bursts, releasing new viral particles which can infect healthy cells
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How you catch a communicable disease

Q What increases the probability of catching a communicable disease?

  • overcrowding& poor ventilation
  • poor health (immunosuppressed e.g. HIV/AIDS) & poor diet (poor immune system)
  • homelessness
  • living / working with migrants from areas where the disease is common

Q What increases the probability of a plant catching a communicable disease?

  • Warmer climates (colder climates destroy spores) & Climate change –increased rainfall & wind / animal vectors enter new areas
  • Sharing contaminated farm equipment & Contaminated water used for irrigation
  • Contaminated manure used as fertiliser
  • Poor plant mineral nutrition
  • Overcrowding (plants grown close together)
  • Some varieties more susceptible / less resistant
  • Damp/warm conditions
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Plant defences against pathogens

Physical defences – Passive

  • Cellulose cell wall & Waterproof and indigestible lignin
  • Waxy cuticle prevents water collecting on leaf surface- absence of water is a passive defence.
  • Stomatal closure – guard cells close the stoma when pathogenic organisms are detected.
  • tylose formation – balloon-like swelling that fills the xylem vessel so it can no longer carry water.  Prevents spread up the plant.  Tylose also contains a high concentration of chemicals (terpenes) toxic to pathogens.
  • Callose – a large polysaccharide that is deposited in the sieve tubes at the end of the growing season.  Deposited around the sieve plates and blocks the flow in sieve tube.  Can prevent the pathogen spreading around the plant.

Chemical defences  – Passive

  • Plant tissues contain a range of chemicals that help to defend against pathogens, some of these are present before infection, but because production of chemicals requires lots of energy, many are not produced until the plant detects an infection.

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Plant defences against pathogens

Physical defences  - Active

When pathogens attack, specific chemicals including specific proteins and glycolipids can be detected by the plant cells.  The plant responds by fortifying the defences already present:

  • Cellulose cell walls become thickened and strengthened with additional cellulose.
  • Callose deposited between plant cell wall and cell membrane near the invading pathogen.  Helps to strengthen the cell wall and blocks the plasmodesmata.
  • Highly reactive oxygen molecules are produced that are capable of damaging the cells of invading organisms.
  • Necrosis – deliberate cell suicide where a few cells are sacrificed to save the rest of the plant. This limits the pathogens access to water and nutrients and prevents it from spreading.  Brought about by intracellular enzymes that are activated by injury – produces brown spots on leaves or dieback.
  • Canker – a sunken necrotic lesion on the woody tissue such as the main stem or branch.  Causes death of the cambium tissue.
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Chemical defences  - Active- DAPHT!!

Defensins –proteins with antimicrobial properties (inhibit transport channels)

Alkaloids – caffeine, nicotine, cocaine, morphine – bitter taste to insects (deter grazing herbivores)

Phenols – antifungal/antibacterial substances such as tannins in bark – prevent attack by insects/cause insects to die

Hydrolytic enzymes-  in spaces between cells e.g. chitinase (digests fungal cell walls), lysosomes (digest bacterial cell walls)

Terpenoids- essential oils – antibacterial and antifungal e.g. menthol, mint, lavender

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Malaria is caused by a single-celled organism called Plasmodium . The organism is transmitted from one person to another by female Anopheles mosquitoes. A mosquito takes up the gametes of the malarial parasite when it feeds on the blood of an infected  person. Fertilisation occurs in the mosquito’s stomach and the immature parasites reproduce. Infective stages of the parasite migrate to the mosquito’s salivary glands. A new person becomes infected when the mosquito takes another meal of blood . The parasites enter the liver of the new victim where further reproduction takes place before migrating to the red blood cells. When an organism, such as the mosquito, is involved in transmission it is called a vector . The malarial parasite can also be transmitted by blood transfusions.

Why has it not been possible to create a vaccine for Malaria?

  • different strains / species / types of Plasmodium
  • therefore different antigens  
  • due to mutation & variation
  • more than one stage in the life cycle within human
  • different stages have different antigens
  • so will need a different vaccine for each strain / stage  
  • parasite concealed / hidden in cells
  • parasite only exposed / in circulation for short time
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How can the spread of malaria be reduced?

reduce mosquito numbers

  • stock ponds with fish (Gambusia) to eat larvae
  • oil on surface
  • spray bacteria (Bacillus thuringiensis) to kill mosquito larvae or DDT / pesticide spray
  • release of sterile male mosquitoes
  • draining ponds / bodies of water

 avoid being bitten by mosquitoes

  • wear insect repellent & long sleeved clothes; sleep under nets
  • nets soaked in insecticide / repellent
  • sleep with pigs / dogs

 use drugs to prevent infection

  • use prophylactic drug / quinine / antimalarial
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Animal diseases


Tuberculosis (TB)- Bacterium- Droplet infection- lungs most often affected, can affect and kill other cells and tissues- Humans/Cattle- BCG vaccine,antibiotics,screen cattle,pasteurise milk.

Bacterial meningitis- Bacterium- sneezing, coughing, kissing, sharing utensils, sharing personal possessions-infection of meninges (membranes surrounding brain and spinal cord) membranes become swollen and may cause brain and nerve damage- Humans- vaccine (against some bacterial strains but not all) antibiotics

HIV/AIDS- Virus- sexual intercourse (semen/ vaginal fluid), sharing contaminated syringes,across placenta/via contaminated breastmilk, contaminated blood transfusions- attack cells of the immune system compromises immune response susceptible to opportunistic infections e.g. TB, pneumonia- Humans- Anti Viral drugs

Influenza- Virus- Droplet infection- attacks respiratory system and causes muscle pain and headaches- Humans, poultry, pigs- Annual vaccine, anti viral drugs.

Athletes foot- Fungus- contaminated surface- itchy skin between toes- humans- anti fungal medications, clean surfaces.

Malaria- Protoctist- vis female anopheles mosquito- parasite in the blood causes headache and fever and may progress to coma and death- Humans- prevent being bitten by mosquitos (insect repellants/nets), kill mosquitoes ( insecticides/ oil on ponds/drain ponds/ release sterile males), prophylactic anti-malarial drugs (quinine/ chloroquine)

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Plant diseases

Name- Type of Pathogen- Symptoms- Organisms Affected

Black sigatoka- Fungus- Leaf spots- Bananas

Ring rot- Bacterium-  Ring of decay in vascular tissue of potato tuber or tomato- Potato, Tomato.

Tobacco Mosaic- Virus- Mottling and discolouration of leaves- Tobacco

Blight- Protoctist- affects leaves and potato tubers- Potato and tomato

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transmission of animal and plant communicable path

Direct Contact-

  • Pathogen transmitted from infected organism to another organism with no intermediary (no vector)
  • e.g. direct physical contact – touching surfaces contaminated with pathogen, exchange of bodily fluids (e.g. blood, semen/vaginal fluid), skin- to –skin contact, puncture wounds (including bites) and sharing needles (HIV/Bacterial meningitis)
  • faecal- oral transmission – eating food or drinking water contaminated by the pathogen (Cholera/ Salmonella)
  • droplet infection – pathogen carried in water droplets in the air when coughing or sneezing (TB / Influenza)
  • spores – can be carried in air or water or reside on surfaces or in the soil (Anthrax /Tetanus)

Indirect Contact-

  • Pathogen transmitted from infected organism to another organism via a vector (a carrier)
  • e.g. protoctist -Plasmodium falciparum which causes Malaria in humans transmitted by female Anopheles mosquito
  •  e.g. fungus  Ophiostoma novo-ulmi  which causes Dutch Elm disease in Elm trees transmitted by  Scolytus multistriatus  beetle
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HIV- Human Immunodeficiency Virus. Attacks T helper cells in the immune system. If the virus is present but inactive then a person is said to be HIV-Positive. List the ways in which HIV can be spread by an infected person:  Unprotected sexual intercourse (via semen and vaginal fluids), Contaminated blood products (blood transfusions), Sharing contaminated intravenous syringes, From mother to baby across placenta, at birth or via contaminated breast milk

 AIDS- Acquired Immune Deficiency Syndrome. Caused by a weakened immune system due to the action of HIV reducing T helper cell numbers.

Explain why people infected with HIV may develop AIDS.

HIV destroys, T helper / T4 cells / T lymphocytes;
HIV replicates and increases in numbers / number of T helper cells decrease;
immune response weakened / body can’t fight disease;
opportunistic infections / named;
body can’t destroy cancer cells / named cancer;
AVP; e.g. detail of action of T helper cells

Explain how HIV affects T lymphocytes.- viral proteins are transcribed and translated because reverse transcriptase in host nucleus, viral DNA is inserted in host DNA, viral mRNA produced to code for viral proteins

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