Transmission of communicable diseases

• Direct transmission

  Pathogen is transferred directly from one individual to another.
• Direct transmission: Direct contact (communicable diseases)


  Kissing or any contact with the body fluids of another person (bacterial meningitis, STIs)

  Direct skin-to-skin contact (ring worm, athelete's foot)

  Microorganisms from faeces transmitted on the hands (diarrhoeal diseases).
• Direct transmission: Inoculation


  Through a break in the skin (during sex-HIV/AIDS)

  From an animal bite (rabies)

  Through a puncture wound or sharing needles (septicaemia).
• Direct transmission: Ingestion

  Taking in contaminated food or drink, or transferring pathogens to the mouth from the hands (amoebic dysentery, diarrhoeal diseases).
• Indirect transmission


  Pathogen travels from one individual to another indirectly.
• Indirect transmission: Fomites

  Inanimate objects such as bedding, socks or cosmetics can transfer pathogens (athlete's foot, gas gangrene and Staphylococcus infections.)
• Indirect transmission: Droplet infection

  Minute droplets of saliva and mucus are expelled from your mouth as you talk, cough or sneeze. If these droplets contain pathogens, when healthy individuals breathe them in they may become infected. (Influenza, Tuberculosis).
• Indirect transmission: Vectors

  A vector transmits communicable pathogens from one host to another. Vectors are often animals (Anopheles Mosquitoes transmit malaria, rat fleas transmit bubonic plague, and dogs, foxes, bats transmit rabies).

  Water also acts as a vector (Diarrhoeal diseases).
• Some communicable can be passed from animals to people, for example bird flu strain H 1 N 1 and Brucellosis (sheep to people). Minimising close contact with animals and washing hands thoroughly following any contact can reduce infection rates. People can also act as vectors for animal diseases (Foot-and-mouth disease).
• Factors affecting the transmission of communicable diseases in animals

  The probability of catching a communicable disease is increased by:

  • Overcrowded living and working conditions
  • Poor nutrition
  • A compromised immune system (having HIV/AIDS or being on immunosuppressant drugs).
  • Poor disposal of waste, providing breeding sites for vectors.
  • Climate change
  • Culture and infrastructure- traditional medical practices can increase transmission.
  • Socioeconomic factors- lack of trained health workers and insufficient public warning when there is an outbreak.
• Transmission of pathogens between plants: Direct transmission

  Direct contact of a healthy plant with any part of a diseased plant (Ring rot, TMV, tomato and potato blight, black sigatoka).
• Transmission of pathogens between plants: Indirect transmission- Soil contamination

  Infected plants often leave pathogens or reproductive spores from protists or fungi in the soil. These can infect the next crop. (Black sigatoka spores, ring rot bacteria, spores of P. infestans and TMV). Some pathogens can survive the composting process so the infection cycle can be completed when contaminated compost is used.
• Transmission of pathogens between plants: Indirect transmission: Vectors

  Humans- pathogens and spores are transmitted by hands, clothing, fomites, farming practices and by transporting plants and crops around the world (TMV survives for years in tobacco products, ring rot survives on farm machinery, potato sacks...).
• Transmission of pathogens between plants: Indirect transmission: Vectors


  Wind- bacteria, viruses and fungal or oomycete spores may be carried in the wind (Black sigatoka blown between Caribbean islands)

  Water- spores swim in the surface film of water on leaves; raindrop splashes carry pathogens and spores (Potato blight spores).

  Animals- Insects and birds carry pathogens and spores from one plant to another as they feed. Insects like aphids inoculate pathogens directly into plant tissues.
• Factors affecting the transmission of communicable diseases in plants

  There are a number of factors:

  • Planting varieties of crops which are susceptible to disease
  • Over-crowding increases the likelihood of contact
  • Poor mineral nutrition reduces resistance of plants.
  • Damp warm conditions increase the survival and spread of pathogens and spores.
  • Climate change- increased rainfall and wind promote the spread of diseases; changing conditions allow animal vectors to spread to new areas; drier conditions may reduce the spread of disease.