Infection and response

• Pathogens

  Microorganisms that cause disease
• Types of pathogens

  • Bacteria
  • Viruses
  • Protists
• Pathogens

  • Can be spread in the air
  • Can be spread in water
• Viruses

  Live and reproduce rapidly inside an organism's cells, which can damage or destroy the cells
• Viruses

  • Measles
  • HIV (human immunodeficiency virus)
  • TMV (tobacco mosaic virus)
• Viruses

  Spread by coughing and sneezing, direct contact with infected people, contact with infected plant material, animal vectors
• Viruses

  Can remain in the soil for decades
• Viruses

  Attack the body's immune system, which can lead to AIDS when the immune system is so damaged that it cannot fight off infections or cancers
• Viruses (in plants)

  Cause mosaic patterns of discolouration or leaves, where chlorophyll is destroyed, reducing the plant's ability to photosynthesize, affecting growth
• Bacteria

  Reproduce rapidly inside organisms and may produce toxins that damage tissues and cause illness
• Bacteria

  • Salmonella
• Bacteria

  Spread by being ingested in or on food
• Gonorrhoea

  Spread by direct sexual contact, a sexually transmitted disease (STD)
• Fungi

  Cause purple or black spots on leaves, which turn yellow and drop early, reducing the plant's ability to photosynthesize, affecting growth
• Protists

  Spread by mosquitoes that feed on infected people and spread the pathogen when they feed on another person
• Communicable diseases is another way of saying infectious diseases
• Herd immunity


  If a large proportion of a population is vaccinated against a disease, the disease is less likely to spread, even if there are some unvaccinated individuals
• Vaccination

  Involves injecting small quantities of dead or inactive forms of a pathogen into the body, stimulating lymphocytes to produce the correct antibodies for that pathogen, so if the same pathogen re-enters the body, the correct antibodies can be produced quickly to prevent infection
• Non-specific defences

  The human body's defences against all pathogens, including the skin, nose, and stomach
• White blood cells

  Fight pathogens, including lymphocytes that produce antitoxins and antibodies, and phagocytes that surround and destroy pathogens
• Antibiotic resistance arises when bacteria, not people, evolve resistance to a drug
• Antibiotics

  Medicines that can kill bacteria in the body, but do not affect viruses
• Drugs that kill viruses often damage the body's tissues, and painkillers only treat the symptoms of viral diseases but do not kill pathogens
• Discovering and developing new drugs


  1. Traditionally extracted from plants and microorganisms, now mostly synthesised by chemists in laboratories

  2. Pre-clinical trials test the drug in cells, tissues, and live animals

  3. Clinical trials extensively test the drug for toxicity, efficacy, and optimal dose, with healthy volunteers and patients, in double-blind trials

  4. Peer review checks the results of clinical trials before publication
• Coronary heart disease (CHD)

  Occurs when the coronary arteries become narrowed by the build-up of fatty material, reducing blood flow and oxygen supply to the heart muscle, which can lead to heart attacks
• Treatments for cardiovascular diseases

  • Stents
  • Statins
  • Heart valve replacement
  • Heart/lung transplant
  • Artificial hearts
• Risk factor

  Any aspect of lifestyle or substance in the body that can increase the risk of a disease developing
• Risk factors and associated diseases

  • Diet (obesity) - Type 2 diabetes
  • Lack of exercise - Cardiovascular diseases
  • Alcohol - Impaired liver function, brain function
  • Smoking - Lung disease, cancers
  • Carcinogens, genetic factors - Cancers
• Cancer

  The result of changes in cells that lead to uncontrolled growth and division, forming malignant tumours that invade neighbouring tissues and spread to other parts of the body
• Treatment of non-communicable diseases linked to lifestyle risk factors can be very costly, both to individuals and to the Government, and high rates of these diseases in a population can be caused by high incidence of lifestyle risk factors