B3 infection and response

• Communicable disease


  A disease caused by a pathogen which can be passed between animals or plants e.g. flu
• Pathogen

  A disease-causing microorganism e.g. bacteria
• How bacteria cause disease

  1. Divide rapidly by binary fission

  2. Kill cells

  3. Produce harmful toxins
• How viruses cause disease

  1. Invade and reproduce inside living body cells

  2. Lead to cell damage
• Ways pathogens can be spread

  • By air - flu, tuberculosis, common cold
  • By water - fungal spores
  • By direct contact - plant diseases, sexually transmitted infections
• Ways to reduce the spread of pathogens

  • Hygiene - handwashing, disinfectants, tissues
  • Reducing contact with infected individuals - quarantine
  • Removing vectors - use of pesticides, removal of habitats
  • Vaccination
• Scientists have not yet developed cures for many viral diseases
• Measles


  A serious viral disease that can cause blindness and brain damage. Main symptoms are fever and red skin rash
• How measles is spread

  By air - through inhalation of droplets from coughs and sneezes
• HIV/AIDS


  HIV is a virus which attacks and damages the immune system until it can no longer function properly. AIDS is the condition resulting from a long-term HIV infection. There is no cure or vaccine for HIV/AIDS
• How HIV is spread

  Direct sexual contact and the exchange of bodily fluids e.g. blood, breastmilk
• How to prevent the spread of HIV/AIDS

  • Use of condoms
  • Screening of blood for transfusions
  • Not sharing needles
  • Bottle-feeding by HIV-positive mothers
  • Use of antiretroviral drugs to prevent the development of AIDS
• Tobacco mosaic virus


  A plant pathogen which causes leaf discolouration when cells are damaged. Affected areas cannot photosynthesise, reducing the crop yield. As there is no treatment, farmers grow TMV-resistant crop strains to avoid infection
• How tobacco mosaic virus is spread

  • Contact between infected and healthy plants
  • Insects may act as vectors which transfer the virus between different plants
• Salmonella


  A type of bacteria found in raw meat, poultry and eggs. If they enter the body via food poisoning, they can affect natural gut bacteria
• Symptoms of salmonella food poisoning

  • Fever
  • Abdominal cramping
  • Vomiting
  • Diarrhoea. May be fatal in very young or elderly populations due the risk of dehydration
• How to limit the spread of salmonella

  • Vaccinating animals intended for consumption
  • Keep raw meat away from cooked meat
  • Disinfect hands and surfaces after contact with raw meat
  • Thoroughly cook meat
• Gonorrhoea


  A sexually transmitted bacterial infection caused by unprotected sex with an infected individual. Early symptoms include yellow/green discharge from genitals and painful urination, although it may be symptomless. Its spread can be controlled through the use of antibiotics (no longer penicillin as many strains are resistant) and using condoms
• Rose black spot

  A fungal disease which causes purple or black spots to develop on rose leaves. It reduces the area of the leaf which is available for photosynthesis and causes leaves to turn yellow and drop prematurely
• How the rose black spot fungus is spread

  Fungal spores are spread by the wind and in water
• How to treat the rose black spot fungus

  • Using fungicides
  • Destroying infected leaves
• Malaria


  Malaria is a disease caused by protist pathogens. The disease is carried from host to host by mosquitoes, and the protists enter the human bloodstream when they feed. Symptoms include fever and shaking, and it may also be fatal in some cases
• How to reduce the spread of malaria

  • Using insecticides
  • Using insect nets to avoid bites
  • Prevent mosquito breeding by removing stagnant water
  • Antimalarial drugs
• How the skin prevents pathogens from entering the body

  • Acts as a physical barrier
  • Scab formation after skin is cut/wounded
  • Antimicrobial secretions which can kill pathogens
  • Healthy skin flora compete with pathogens and act as an additional barrier
• How the respiratory system prevents pathogens from entering the body

  • Nose - has hairs and mucus which trap pathogens
  • Trachea and bronchi - have mucus that traps pathogens. Ciliated cells move mucus to the mouth so it can be swallowed
• How the stomach prevents pathogens infecting the body

  Secretes hydrochloric acid - kills any pathogens present
• How phagocytosis protects us against disease

  White blood cells (phagocytes) ingest and destroy pathogens so they cannot infect more cells
• Nose


  • Has hairs and mucus which trap pathogens
• Trachea and bronchi


  • Have mucus that traps pathogens
  • Ciliated cells move mucus to the mouth so it can be swallowed
• Stomach

  Secretes hydrochloric acid - kills any pathogens present
• Aphids


  Use their sharp mouthparts to extract sap from the plant phloem, weakening the plant. They are also vectors who can transfer pathogens from diseased plants to healthy plants.
• Reducing the number of aphids

  1. Chemical pesticides

  2. Biological pest control - using ladybirds
• Nitrate ions


  Required to convert sugars into proteins which are required for the plant to grow. If there is not an adequate supply, the growth of the plant will be stunted.
• Magnesium ions

  Required to synthesise chlorophyll, which absorbs light energy during photosynthesis. If there is not an adequate supply, the leaves cannot photosynthesise properly and they turn yellow (chlorosis).
• Symptoms of disease in plants

  • Stunted growth (lack of nitrate ions)
  • Spotted leaves (eg. rose black spot fungus)
  • Decay/rotting
  • Growths
  • Malformation of stems/leaves
  • Discolouration
  • Presence of pests
• Ways to identify plant disease

  • Reference to a gardening website or manual
  • Laboratory testing
  • Monoclonal antibody test kits
• Physical defence responses used by plants

  • Cellulose cell walls
  • Tough waxy cuticle on leaves
  • Outside layer on stems/bark on trees
  • Leaf fall
• Chemical defence responses used by plants

  • Antibacterial chemicals
  • Poisons
• Mechanical defence responses used by plants

  • Thorns and hairs to deter animals
  • Leaves that droop/curl on contact
  • Mimicry to trick animals
• How do antibody production protect us against disease?

  White blood cells produce antibodies which are complementary to a specific antigen on a pathogen. The binding of antibodies to antigens causes pathogens to clump together, making them easier to destroy. In the case of a second infection, the correct antibodies can be produced rapidly, preventing the person from getting the same disease again