Infection and response

• Pathogens


  Microorganisms that cause infectious disease
• Types of pathogens


  • Viruses
  • Bacteria
  • Protists
  • 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 the destruction of the cells makes the individual feel ill
• Bacteria

  • Small
  • They multiply very quickly through dividing by a process called binary fission
  • They 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

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

  • Direct contact
  • By water
  • By air
• Direct contact

  Touching contaminated surfaces, kissing, contact with bodily fluids, direct skin to skin, microorganisms from faeces, infected plant material left in field
• By water

  Drinking or coming into contact with dirty water
• By air

  Pathogens can be carried in the air and then breathed in (a common example is the droplet infection, which is when sneezing, coughing or talking expels pathogens in droplets which can be breathed in)
• Ways to reduce the damage of disease

  • Improving hygiene
  • Reducing contact with infected individuals
  • Removing vectors
  • Vaccination
• Improving hygiene

  Hand washing, using disinfectants, isolating raw meat, using tissues and handkerchiefs when sneezing
• Removing vectors


  Using pesticides or insecticides and removing their habitat
• Vaccination

  Injecting a small amount of a harmless pathogen into an individual's body, they can become immune to it so it will not infect them. This means they 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
• Viruses are particularly dangerous as they can enter all types of cells, and scientists are yet to develop medicines to cure them
• Measles

  • Symptoms: Fever and red skin rash, can lead to other problems such as pneumonia (lung infection), encephalitis (brain infection) and blindness
  • How it is spread: Droplet infection
  • How it is being prevented: Vaccinations for young children to reduce transmission
• HIV

  • Symptoms: Initially flu-like symptoms, then the virus attacks the immune system and leads to AIDS (a state in which the body is susceptible to many different diseases)
  • How it is spread: By sexual contact or exchange of bodily fluids such as blood
  • How it is being prevented: Using condoms, not sharing needles, screening blood when it is used in transfusions, mothers with HIV bottle-feeding their children instead of breastfeeding, use of antiretroviral drugs (stop the virus replicating in the body)
• Tobacco mosaic virus

  • Symptoms: Discolouration of the leaves, the affected part of the leaf cannot photosynthesise resulting in the reduction of the yield
  • How it is spread: Contact between diseased plants and healthy plants, insects act as vectors
  • How it is being prevented: Good field hygiene and pest control, growing TMV-resistant strains
• Salmonella food poisoning

  • Symptoms: Fever, stomach cramps, vomiting, diarrhoea (all caused by the toxins they secrete)
  • How it is spread: These bacteria can be found in raw meat and eggs, unhygienic conditions
  • How it is being prevented: Poultry are vaccinated against Salmonella, keeping raw meat away from cooked food, avoid washing it, wash hands and surfaces when handling it, cook food thoroughly
• Gonorrhoea

  • Symptoms: Thick yellow or green discharge from the vagina or penis, pain when urinating
  • How it is spread: It is a sexually transmitted disease spread through unprotected sexual contact
  • How it is being prevented: By using contraception such as condoms and antibiotics (used to be treated with penicillin but many resistant strains are developing)
• Rose black spot

  • Symptoms: Purple or black spots on leaves of rose plants, reduces the area of the leaf available for photosynthesis, leaves turn yellow and drop early
  • How it is spread: The spores of the fungus are spread in water (rain) of by wind
  • How it is being prevented: By using fungicides or stripping the plant of affected leaves (have to be burnt)
• Malaria

  • Symptoms: Fevers and shaking (when the protists burst out of blood cells)
  • How it is spread: The vector is the female Anopheles mosquito, in which the protists reproduce sexually. When the mosquito punctures the skin to feed on blood, the protists enter the human bloodstream via their saliva.
  • How it is being prevented: Using insecticide coated insect nets while sleeping, removing stagnant water to prevent the vectors from breeding, travellers taking antimalarial drugs to kill parasites that enter the blood
• Non-specific defence system


  • The skin
  • The nose
  • The trachea and bronchi
  • The stomach
• The skin


  • Acts as a physical barrier
  • It produces antimicrobial secretions to kill pathogens
  • Good microorganisms known as skin flora compete with the bad microorganisms for space and nutrients
• The nose

  Has hairs and mucus (sticky substance) which prevent particles from entering your lungs
• The trachea and bronchi


  • Secrete mucus in order to trap pathogens
  • Cilia (hair-like structures on cells) beat to waft mucus upwards so it can be swallowed
• The stomach

  Produces hydrochloric acid that kills any pathogens in your mucus, or food and drink
• Ways the specific immune system protects you

  • Phagocytosis (engulfing and consuming pathogens)
  • Producing antibodies
  • Producing antitoxins
• Phagocytosis


  This destroys pathogens, meaning they can no longer make you feel ill
• Producing antibodies

  • Each pathogen has an antigen on their surface, which is a structure which a specific complementary antibody can bind to. Once antibodies begin to bind to the pathogen, the pathogens start to clump together, resulting in it being easier for white blood cells to find them.
  • If you become infected again with the same pathogen, the specific complementary antibodies will be produced at a faster rate. The individual will not feel the symptoms of the illness. They are said to be immune.
• Producing antitoxins

  They neutralise the toxins released by the pathogen by binding to them
• Vaccination

  • The vaccine contains a dead or inactivated form of the pathogen
  • This stimulates white blood cells to produce antibodies complementary to the antigens on the pathogen
• Antibiotics

  • Medicines that kill bacterial pathogens inside the body, without damaging body cells
  • They cannot kill viruses as they use body cells to reproduce, meaning any drugs that target them would affect body tissue too
  • They 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
  • Their use has decreased the number of deaths from bacterial diseases
  • An example is Penicillin
• Painkillers

  Only treat the symptoms of the disease, rather than the cause
• The great concern is that bacteria are becoming resistant to antibiotics
• Antibiotic resistance

  • Mutations can occur during reproduction resulting in certain bacteria no longer being killed by antibiotics
  • When these bacteria are exposed to antibiotics, only the non-resistant one die
  • The resistant bacteria survive and reproduce, meaning the population of resistant bacteria increases
  • This means that antibiotics that were previously effective no longer work
• Ways to prevent antibiotic resistance

  • Stop overusing antibiotics
  • Finishing courses of antibiotics to kill all of the 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)