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

  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.
• 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
  • Secretes antimicrobial substances which kills pathogens
  • Trachea and Bronchi secrete mucus to trap pathogens
  • Trachea and Bronchi are lined with cilia which waft the mucus up to the back of the throat where its swallowed
  • Stomach produces hydrochloric acid
• 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
• 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
• Antibiotics have no effect on viral pathogens as they live inside the host's (human) cells. Therefore, it is difficult to design drugs that would kill the virus and not destroy human cells at the same time
• Antibiotic resistance

  • Antibiotic resistance occurs when mutations lead to individual bacteria being resistant to an antibiotic
  • These bacteria are able to survive, reproduce and pass on their alleles, leading to a greater proportion of antibiotic-resistant bacteria
  • This is concerning as some types of bacteria are becoming resistant to all known antibiotics, so the diseases that they cause cannot be cured
• How to prevent antibiotic resistance

  • Avoid overuse and unnecessary use of antibiotics - eg. antibiotics are often used for viral infections
  • Finish antibiotic courses - to ensure all bacteria is killed
• Painkillers can only treat the symptoms but do not kill pathogens
• Digitalis

  Heart drug extracted from foxgloves
• Aspirin

  Painkiller originating from a compound found in willow bark