Topic 3 - Infection and Response

• Pathogens


  Microorganisms that cause infectious disease, including viruses, bacteria, protists and fungi
• Viruses


  • Very small
  • Move into cells and use the biochemistry to make many copies of itself
  • This leads to the cell bursting and releasing all of the copies into the bloodstream
  • The damage and destruction of the cells makes the individual feel ill
• Bacteria


  • Small
  • Multiply very quickly through dividing by a process called binary fission
  • Produce toxins that can damage cells
• Protists


  • Some are parasitic, meaning they use humans and animals as their hosts (live on and inside, causing damage)
• Fungi


  • Can be single celled or have a body made of hyphae (thread-like structures)
  • Can produce spores which can be spread to other organisms
• Ways pathogens are spread

  • Direct contact (touching contaminated surfaces, kissing, contact with bodily fluids, microorganisms from faeces, infected plant material)
  • By water (drinking or coming into contact with dirty water)
  • By air (pathogens carried in droplets from sneezing, coughing or talking)
• Ways to reduce the spread of disease

  • Improving hygiene (hand washing, using disinfectants, isolating raw meat, using tissues and handkerchiefs when sneezing)
  • Reducing contact with infected individuals
  • Removing vectors (using pesticides or insecticides and removing their habitat)
  • Vaccination
• Vaccination


  Injecting a small amount of a harmless pathogen into an individual's body, so they become immune to it and cannot pass it on
• Herd immunity


  When a large proportion of the population is vaccinated, the spread of the pathogen is reduced as there are less people to catch the disease from
• Measles


  • Symptoms: Fever and red skin rash, can lead to other problems such as pneumonia, encephalitis and blindness
  • Spread by droplet infection
  • Prevented by vaccinations for young children
• HIV


  • Symptoms: Initially flu-like, then virus attacks the immune system leading to AIDS
  • Spread by sexual contact or exchange of bodily fluids
  • Prevented by using condoms, not sharing needles, screening blood, mothers with HIV bottle-feeding
  • Treated with antiretroviral drugs
• Tobacco mosaic virus


  • Symptoms: Discolouration of leaves, affected part cannot photosynthesise resulting in reduced yield
  • Spread by contact between diseased and healthy plants, insects act as vectors
  • Prevented by good field hygiene, pest control, growing TMV-resistant strains
• Salmonella food poisoning


  • Symptoms: Fever, stomach cramps, vomiting, diarrhoea
  • Spread through raw meat, eggs, unhygienic conditions
  • Prevented by vaccinating poultry, keeping raw and cooked food separate, cooking thoroughly
• Gonorrhoea


  • Symptoms: Thick yellow/green discharge, pain when urinating
  • Spread through unprotected sexual contact
  • Prevented by using contraception, treated with antibiotics (some resistant strains developing)
• Rose black spot


  • Symptoms: Purple/black spots on leaves, reduces photosynthesis, leaves turn yellow and drop early
  • Spread by spores in water or wind
  • Prevented by using fungicides or stripping affected leaves
• Malaria


  • Symptoms: Fevers and shaking
  • Spread by female Anopheles mosquito, which the protist reproduces in
  • Prevented by insecticide nets, removing stagnant water, antimalarial drugs for travellers
• Non-specific defences


  • Skin (physical barrier, antimicrobial secretions, skin flora)
  • Nose (hairs, mucus)
  • Trachea and bronchi (mucus, cilia)
  • Stomach (hydrochloric acid)
• Specific immune system responses

  • Phagocytosis (engulfing and consuming pathogens)
  • Producing antibodies (bind to pathogen antigens)
  • Producing antitoxins (neutralise pathogen toxins)
• Antibiotics

  Medicines that kill bacterial pathogens inside the body, without damaging body cells
• Antibiotics cannot kill viruses as they use body cells to reproduce
• Painkillers


  Only treat the symptoms of a disease, rather than the cause
• Bacteria are becoming resistant to antibiotics due to mutations during reproduction
• Ways to prevent antibiotic resistance

  • Stop overusing antibiotics
  • Finish courses of antibiotics to kill all bacteria
• Many drugs were initially discovered in natural sources
• Epidemics (lots of cases in an area) can be prevented through herd immunity
• Bad reactions (such as fevers) can occur in response to vaccines (although very rare)
• The great concern is that bacteria are becoming resistant to antibiotics
• Mutations can occur during reproduction resulting in certain bacteria no longer being killed by antibiotics


  1. When these bacteria are exposed to antibiotics, only the non-resistant one die

  2. The resistant bacteria survive and reproduce, meaning the population of resistant bacteria increases

  3. This means that antibiotics that were previously effective no longer work
• To prevent the development of these resistant strains

  • Stop overusing antibiotics- this unnecessarily exposes bacteria to the antibiotics
  • Finishing courses of antibiotics to kill all of the bacteria
• Toxicity


  How poisonous a drug is
• Efficacy


  How well a drug carries out its role
• Preclinical testing


  Using cells, tissues and live animals to test new drugs
• Clinical trials


  Using volunteers and patients to test new drugs
• New drugs today are mainly synthesised by chemists
• Plants

  • Aspirin (originates from willow)
  • Digitalis (originates from foxgloves)
• Discovery of Penicillin


  1. Alexander Fleming was growing bacteria on plates

  2. He found mould (Penicillium mould) on his culture plates, with clear rings around the mould indicating there was no longer any bacteria there

  3. He found that the mould was producing a substance called penicillin, which killed bacteria
• Placebo


  Appears to look like the drug but has no active ingredient so no effect
• Single-blind

  Only the doctor knows whether the patient is receiving the drug
• Double blind

  Neither the patient or doctor knows whether they are receiving the drug, removing any biases the doctor may have when they are recording the results
• The results then need to be peer reviewed by other scientists to check for repeatability