Unit 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 instead of breastfeeding
  • 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 food thoroughly
• Gonorrhoea


  • Symptoms: Thick yellow/green discharge, pain when urinating
  • Spread through unprotected sexual contact
  • Prevented by using contraception, treated with antibiotics (but 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 sexually
  • Prevented by using insecticide nets, removing stagnant water, taking antimalarial drugs
• Non-specific defence system


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


  • Phagocytosis (white blood cells 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, meaning any drugs that target them would affect body tissue too
• Painkillers


  Only treat the symptoms of a disease, rather than the cause
• Antibiotics can be taken as a pill, syrup or directly into the bloodstream
• Different antibiotics are effective against different types of bacteria, so receiving the correct one is important
• The use of antibiotics has decreased the number of deaths from bacterial diseases
• Bacteria are becoming resistant to antibiotics due to mutations that occur during reproduction
• Ways to prevent antibiotic resistance

  • Stop overusing antibiotics
  • Finish courses of antibiotics to kill all bacteria
• 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 drugs
• Clinical trials


  Using volunteers and patients to test drugs
• New drugs being developed need to be tested to ensure they are safe and effective
• Preclinical testing is first done on healthy volunteers with a low dose to ensure there are no harmful side effects
• Drugs are then tested on patients to find the most effective dose
• To test how well a drug works, patients are split into two groups with one group receiving the drug and one receiving a placebo (appears to look like the drug but has no active ingredient so no effect) so the effect of the new drug can be observed
• Single-blind


  Only the doctor knows whether the patient is receiving the drug