Communicable diseases

• 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

  Once inside the body, they divide rapidly by binary fission. They kill cells and produce harmful toxins.
• How viruses cause disease

  They invade and reproduce inside living body cells, leading to cell damage.
• Ways pathogens can be spread

  • By air - flu, tuberculosis and the common cold are spread by droplet infection
  • By water - fungal spores in water spread plant diseases
  • By direct contact - common in plant diseases and 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 and insecticides, removal of habitats
  • Vaccination
• Scientists have not yet developed cures for many viral diseases, so it is especially important to prevent the spread of viral diseases
• Measles


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

  By air - through the 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.
• Ways 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 (TMV)


  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
• Ways 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 the rose black spot fungus can be treated

  • 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.
• Ways 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
• How antibody production protects us against disease

  Antibodies bind to pathogens, marking them for destruction by other immune 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
• Phagocytosis


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


  • 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 getting the same disease again
• Antitoxin production


  Antitoxins bind to toxins released by pathogens and neutralise them
• Vaccination


  • Contains a dead or inactivated form of the pathogen which stimulates white blood cells to produce complementary antibodies to the pathogen
  • In the case of a second infection, memory cells can rapidly produce the correct antibodies and prevent illness
• Herd immunity


  If a sufficiently high proportion of a population are immune to a disease (especially through vaccination), the spread of this disease will be limited
• Advantages of vaccinations


  • They have eradicated many deadly diseases eg. smallpox
  • Many epidemics can be prevented by vaccinations
  • Herd immunity protects those who cannot have vaccinations
• Disadvantages of vaccinations


  • Not guaranteed to work - might not protect against multiple strains of a pathogen
  • May be side effects or adverse reactions
• Antibiotics


  They can kill bacterial pathogens inside the body
• How antibiotics work


  • Antibiotics eg. penicillin kill bacterial pathogens inside the body, but do not kill human cells
  • Whilst some antibiotics kill a wide range of bacteria, it is important that the right antibiotic is used for specific bacteria