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Communicable Diseases, Disease Prevention and the Immune System
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This is a document that consists of notes for the topic, Communicable Diseases, Disease Prevention and the Immune System, as part of the AS/A level OCR Biology A course

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  • May 27, 2023
  • 8
  • 2021/2022
  • Other
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Disease - a condition that impairs the normal functioning of an organism

Pathogen - an organism that causes disease (types - bacteria, fungi, viruses + Protoctista
[type of single-celled eukaryotic organism])

Communicable disease - a disease that can be spread between organisms

Diseases:

● Tuberculosis (TB) - affects animals, typically humans and cattle - Bacterium
● Bacterial Meningitis - affects humans - Bacterium
● Ring rot - affects potatoes, tomatoes - Bacterium

● HIV/AIDS - affects humans - Virus
● Influenza - affects animals, including humans - Virus
● Tobacco Mosaic Virus - affects plants - Virus

● Black sigatoka - affects banana plants - Fungus
● Ringworm - affects cattle - Fungus
● Athlete’s foot - affects humans - Fungus

● Potato/tomato late blight - affects potatoes/tomatoes - Protoctist
● Malaria - affects animals, including humans - Protoctist

Direct transmission - when a disease is transmitted directly from one organism to another
Happen in several ways - droplet infection (coughing/sneezing tiny droplets of mucus),
sexual intercourse, or touching an infected organism

Examples:
● HIV can be transmitted directly between humans via sexual intercourse
● Athlete’s foot can be spread via touch

Indirect transmission - when a disease is transmitted from one organism to another via an
intermediate (air, water, food or another organism [vector])

Examples:
● Potato/tomato late blight is spread out when spores are carried out between plants
(first in the air, then in the water)
● Malaria is spread between humans (and other animals) via mosquitoes - the vector -
don’t cause malaria but carries the protoctista that cause it

Living conditions, climate and social factors affect disease transmission
Overcrowded living conditions increase the transmission of many communicable diseases:

Example
● TB is spread directly via droplet infection - also spread indirectly because the
bacteria can remain in the air for a long time. The risk of TB is increased when it is
crowded

, Climate Example:
● Potato/tomato late blight is common during wet summers - spores need water to
spread
● Malaria is most common in tropical countries (hot + humid) - ideal conditions for
mosquitoes to breed

Social factor example:
Risk of HIV infection is high in places where there’s limited access to:
● Good healthcare - people less likely to be diagnosed + treated, and the most
effective anti-HIV drugs are less likely to be available, so virus more likely to be
passed on to others
● Good health educations - inform people about how HIV is transmitted and how it can
be avoided

Primary, non-specific defences in animals:

Skin - acts as physical barrier, blocking pathogens from entering. Also a chemical barrier by
producing chemicals that are antimicrobial + can lower pH, inhibiting growth of pathogens

Mucous membranes - protect body openings that are exposed to the environment. Some
membranes secrete mucus - sticky substance that traps pathogens + contains antimicrobial
enzymes

Blood clotting - a mesh of protein (fibrin) fibres - they plug wounds to prevent pathogen entry
and blood loss - formed by series of chemical reactions that take place when platelets are
exposed to damaged blood vessels

Inflammation - swelling, pain, heat and redness triggered by tissue damage - tissue releases
molecules - increases blood vessel permeability - start to leak fluid into surrounding area.
Molecules also cause vasodilation (widening of blood vessels) - increase blood flow to
affected area - makes area hot and brings WBCs to fight any pathogens

Wound repair - skin can repair itself and reform a barrier against pathogen entry - repaired
by outer layer of skin cells dividing and migrating to edges of wound. Tissue below contracts
to bring edges of wound closer together. Then it is repaired using collagen fibres - too many
and you will get a scar

Expulsive reflexes (coughing/sneezing) - sneeze happens when the mucous membranes in
nostrils are irritated by dust or dirt. A cough stems from irritation in the respiratory tract

Plant - Physical defences against pathogens

Most plant leaves + stems have a waxy cuticle - provides physical barrier against pathogen
entry - may stop water collecting on leaf - reduce risk of infection by pathogens transferred
between plants in water

Plant cells surrounded by cell walls - form a physical barrier against pathogens that go past
the waxy cuticle

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