topic 3- infection and response

• Pathogens

  Microorganisms that enter the body and cause disease
• Types of pathogens

  • Bacteria
  • Viruses
  • Protists
  • Fungi
• Bacteria


  • Very small living cells
  • Can reproduce rapidly inside your body
  • Live inside your cells and replicate themselves using the cells' machinery
• Viruses

  • Not cells, much smaller than bacteria
  • Replicate inside host cells, causing cell damage and making you ill
• Protists


  • Single-celled eukaryotes, some are parasites that can be transferred by vectors
• Fungi


  • Can be single-celled or have a body made up of hyphae (thread-like structures)
  • Hyphae can grow and penetrate human skin and plant surfaces, causing disease
  • Hyphae can produce spores that can spread to other plants and animals
• Ways pathogens can be spread

  • Water
  • Air
  • Direct contact
• Measles


  • Viral disease spread by droplets from an infected person's sneeze or cough
  • Causes red skin rash and fever
  • Can lead to serious complications like pneumonia or encephalitis
• HIV

  • Viral disease spread through sharing of needles or contact with infected bodily fluids
  • Initially causes flu-like symptoms, then no symptoms for years as it attacks the immune system
  • Late stage is known as AIDS when the immune system is badly damaged
• Tobacco mosaic virus (TMV)


  Virus that affects many plant species, causing a mosaic pattern on leaves and reducing photosynthesis
• Rose black spot

  • Fungal disease that causes purple/black spots on rose leaves, reducing photosynthesis and plant growth
  • Spreads through water or wind, can be treated with fungicides
• Malaria


  • Disease caused by a protist, part of whose life cycle takes place in mosquitoes (vectors)
  • Causes recurring fever episodes, can be fatal
  • Spread can be reduced by stopping mosquito breeding and using insecticides/nets
• Salmonella


  • Bacterial disease that causes food poisoning with symptoms like fever, cramps, vomiting and diarrhoea
  • Caused by eating contaminated food, most poultry is vaccinated against it in the UK
• Gonorrhoea


  • Sexually transmitted bacterial disease causing pain when urinating and discharge
  • Originally treated with penicillin but now strains are resistant, so antibiotics and barrier contraception are used to prevent spread
• Ways to reduce/prevent disease spread

  • Being hygienic
  • Destroying vectors
  • Isolating infected individuals
  • Vaccination
• Human body defences

  • Skin, mucus, cilia, stomach acid act as physical/chemical barriers
  • White blood cells can consume pathogens, produce antibodies, and neutralise toxins
• Vaccination

  • Involves injecting small amounts of dead/inactive pathogens to stimulate antibody production
  • Provides immunity so white blood cells can rapidly respond to future infections
• Pros of vaccination

  • Helped control many communicable diseases
  • Can prevent epidemics if high vaccination rates
• Cons of vaccination

  • Vaccines don't always work
  • Rare risk of bad reactions
• Symptom-relieving drugs

  Don't cure the underlying cause, just reduce symptoms (e.g. painkillers)
• Curative drugs (antibiotics)


  • Kill or prevent growth of bacteria causing the problem
  • Different antibiotics are effective against different bacteria
• Antibiotics and viruses

  Antibiotics don't work against viruses as they reproduce using host cells
• Antibiotic resistance

  • Bacteria can mutate to become resistant to antibiotics
  • Resistant strains survive and reproduce, reducing antibiotic effectiveness
  • Overuse of antibiotics speeds up resistance development
• Bacteria can mutate, sometimes the mutations cause them to be resistant to (not killed by) antibiotics
• If you have an infection, some of the bacteria might be resistant to antibiotics
• When you treat the infection, only the non-resistant strains of bacteria will be killed
• The individual resistant bacteria will survive and reproduce, and the population of the resistant strain will increase
• This resistant strain could cause a serious infection that can't be treated by antibiotics
• MRSA (meticillin-resistant Staphylococcus aureus) causes serious wound infections and is resistant to the powerful antibiotic metiollin
• To slow down the rate of development of resistant strains, it's important for doctors to avoid over-prescribing antibiotics
• It's also important that you finish the whole course of antibiotics and don't just stop once you feel better
• Plants produce a variety of chemicals to defend themselves against pests and pathogens
• Some of these plant chemicals can be used as drugs to treat human diseases or relieve symptoms
• A lot of our current medicines were discovered by studying plants used in traditional cures
• Drugs discovered from plants

  • Aspirin
  • Digitalis
• Some drugs were extracted from microorganisms, for example penicillin was discovered from the Penicillium notatum mould
• Nowadays, drugs are made on a large scale in the pharmaceutical industry, they're synthesised by chemists in labs
• Before new drugs can be given to the general public, they have to go through a thorough testing procedure
• Stages in drug testing

  1. Preclinical testing on human cells and tissues

  2. Preclinical testing on live animals

  3. Clinical trials on human volunteers
• The law in Britain states that any new drug must be tested on two different live mammals