4.1 Biology Communicable diseases

• Infectious Disease

  • Diseases caused by a pathogen that passes from infected individuals to uninfected individuals
  • Also known as communicable diseases
  • Examples are Cholera, HIV, malaria, tuberculosis
• Non-Infectious Diseases

  • Long-term diseases that are not caused by pathogens
  • Examples are Lung Cancer, CHD, depression
• Types of Pathogens

  • Bacteria
  • Viruses
  • Fungi
  • Protoctista
• Bacteria

  • remain within the body cavities or spaces
• Tuberculosis

  • Caused by bacteria
  • Infect the lungs causing coughing and mucus
  • Caused by poor hygiene + sanitation
• Bacterial Meningitis

  • Bacterial infection spread to the brain causing blood poisoning + rapid death
  • Transmitted through direct - airborne droplets
  • Causes blotchy rashes
  • Cured by antibiotics if delivered early enough
• Ring Rot

  • Caused by bacteria
  • Causes ring of decay in the tissue of potato tubers or tomatoes accompanied by leaf wilting
  • Spread via contact with other infected tubers
  • No cure
  • Infected field cannot be used for about 2 years
• HIV/AIDS

  • Caused by a virus
  • Gradually destroys the immune system so you become susceptible to other diseases
  • Virus is passed through bodily fluids like through unprotected sex, shared needles + passing from mother to offspring
  • No cure for HIV/AIDS however anti-retroviral drugs can slow the progress of the disease
• Influenza

  • Caused by virus
  • Attacks the respiratory system causing muscle pains + headaches
  • Kills ciliated epithelium cells leaving airways susceptible to secondary infections
  • Fatal for younger children, the elderly and those who have chronic illnesses
  • Every so often there is a major change in the surface of the antigen meaning there may be no memory cells available for secondary immunity leading to pandemics
  • Annual flu vaccine + no cure
• Tobacco Mosaic Virus

  • Caused by virus
  • Infects tobacco plants
  • Damages leaves, flowers + fruit
  • Stunts growth and reduces yield
  • Resistant crop strains are available for management of disease however there is no actual cure
  • Spread via contact with leaves of other infected plants and also through vectors
• Black sigatoka

  • Caused by Fungi
  • Causes spots on leaves of banana plants
  • Hyphae penetrate + digests the cells which turn the leaves black
  • Resistant strains are being developed
  • Fungicide treatment can control the spread but there is no cure
  • Spread vis spores
• Ringworm

  • Caused by fungi
  • Affects many mammals
  • Different fungi infect different species
  • Causes grey/white infectious circular areas of skin
  • Not damaging but can be itchy
  • Antifungal creams are effective cures
• Athlete's Foot

  • Fungi
  • Caused by form of ringworm
  • Grows + digests warm and moist skin between toes
  • Causes cracking which can become sore
  • Antifungal creams is a good cure
• Blight (Tomatoes + potatoes)

  • Protoctista
  • Affects the leaves and potato tubers
  • Spread via spores
  • Hyphae penetrates the host cells
  • No cure but resistant strains and careful management
  • Chemical treatments can reduce the risk of infection
• Malaria

  • Protoctista
  • Spread by the bite of infected mosquito
  • Reproduce inside female mosquitos
  • Makes people vulnerable other infections
  • No vaccine and limited cures however prevention methods are effective
• Disease Transmission 


  Transfer of pathogens from an infected host to an uninfected host
• Types of disease transmission

  • Direct - from one host to another
  • Indirect - a second organism (vector) that is unaffected by the pathogen transfers it to a new host
• Direct Transmission

  • Physical Contact
  • Being in close proximity via air/droplets
  • Spores
• Indirect Transmission

  • Vectors - organism that transfers a pathogen form an infected individual to an uninfected organism
  • Insects
• Factors affecting disease transmission

  • Presence of pathogens
  • Presence of susceptible individuals
  • Animals and plants can be resistant to some diseases
  • Immunity
  • Proportion of resistant or immune individuals in a population - the higher the proportion the lower the probability of transmission
• Factors affecting Indirect Transmission

  • Affected by the biology of the vectors involved
  • Population of vectors influenced by the weather and climate
• Factors Affecting the Transmission of Human Diseases

  • Poverty
  • Standard of Water
  • Level of human movement and migration
  • Behaviour or cultural practices of humans
• Passive Defence Mechanism 


  Always present in plants like:

  Like physical barriers preventing the pathogen from entering

  Some are chemicals which reduce/prevent the growth of pathogens
• Active Defence Mechanisms 


  Mechanisms which are activated when pathogens invade

  Hypersensitivity

  Formation of callose
• Types of Passive Defence Mechanisms (Physical)

  • Waxy Cuticle
  • Cellulose Cell Wall
  • Closed stomata
  • Bark
• Types of Passive Defence Mechanisms (Chemical)

  • Prevent the pathogens from growing on the surface of the plant by creating acidic conditions
  • Toxic Compounds
  • Sticky resin
  • Compounds to encourage growth of competing microbes
  • Alkaloids
• Types of Active Defences

  • Increased production of physical barriers like the callose blocks in phloem and lignin in the xylem cell walls
  • Increases production of chemicals
  • Hypersensitivity - death of tissues
  • Cell Signalling
• First Line of Defence (Mammals)

  • Prevents the entry of pathogens
  • Skin
  • Mucous membranes
  • Expulsive reflexes
  • Chemical secretions
• Skin as a first line defence

  • Outer layer of dry and dead cells filled with keratin which is a tough fibrous protein
  • Acts as a physical barrier
  • Secretions of sebum
  • Evaporation of sweat from skin leaves back a salt residue
  • Lack of moisture, low pH + high salinity creates an inhospitable environment for growth of microorganisms
• Mucous Membranes

  • Line the gut, airways and reproductive systems
  • Consist of epithelial cells + mucus secreting cells like goblet cells
  • Mucus is sticky which traps viruses + bacteria
• Expulsive Reflexes

  • When pathogens irritate the lining of the airways it can trigger coughing or sneezing
• Second Line Defence (Mammals)

  • When pathogens manage to evade the first line of defence the second line will respond
  • Involves the phagocytic cells + antimicrobial proteins responding to the invading pathogens
  • Include: Blood clotting, inflammation, wound repair, phagocytosis
• Blood Clotting

  • Stops loss of blood
  • Prevents entry of pathogens
  • Provides a protective surface for wound healing underneath
• Steps of Blood Clotting

  1. Damage to blood vessel wall which releases collagen + clotting factors
  2. Platelets bind to collagen releasing clotting factors which forms temporary plugs
  3. Inactive thrombokinase is turned into active thrombokinase activated by the clotting factors
  4. Thrombokinase and Ca2+ convert prothrombin to thrombin
  5. Thrombin converts fibrinogen to fibrin
  6. Fibrin fibres attach to platelets in plug
  7. RBC and platelets are trapped in fibrin net which forms a clot
• Inflammation 


  Mast cells secrete histamines which attract white blood cells towards them
• Purpose of Inflammation

  • Local response to tissue damage
  • Release of chemical signalling molecules
  • Increase in blood flow
  • Movement of phagocytes into the infected tissues
• Symptoms of Inflammation

  • Redness - Due to increase in blood flow allowing more phagocytes getting to the infected area
  • Pain - signal is sent to the brain
  • Heat - Due to increase in blood flow
  • Swelling - Due to increase tissue fluid build up as capillaries become more leaky and more water gets leaked out into the tissues along with the phagocytes
• Steps of Inflammation

  1. Vasodilation/increases vessel permeability
  2. Phagocyte migration
  3. Tissue repair
• Wound Repair 


  Stem cells divide + platelets secrete growth factors

  Stem cells under a scab begin to divide to repair the wound

  Platelets secrete growth factors that attract cells to divide + grow
• Phagocytes

  • White Blood Cells which are produced continuously in the bone marrow
  • Responsible for removing dead cells and invasive microorganisms
  • Main types: Neutrophils, Macrophages, Dendritic cells
  • Carry out phagocytosis which is the process of recognising and engulfing a pathogen