HomeCells & MoleculesDiseasesEcosystemHuman BiologyAQA BIOL1AQA HBIO1AQA HBIO2AQA HBIO4AQA HBIO5
HBIO2 > Parasites
Welcome, Guest!
Login with Facebook | Login


  • Parasites (endo-) or (ecto-) feed on living organisms (host) while causing harm
  • They gain benefits from them (e.g. unlimited supply of nutrients, H2O, constant temp)
  • Can be bacteria, protoctists, viruses, fungi, arthropods, platyhelminthes


  • Modification of mouthparts and digestive enzymes
    • Allow attachment to host
    • Allow use of host's food supply, blood or tissues
  • Reduction of unnecessary sensory organs and locomotory organs in adult stage
    • Not required as they live in protected, optimum conditions
  • Specialised reproductive strategies
    • May have male and female reproductive systems
    • May reproduce asexually
    • May produce large amounts of eggs/larvae
  • Resist attack by immune system
    • Inaccessible to immune system (gut)
    • Adapt antigens from host (host recognizes parasite as own tissue)
    • Change its own antigens
  • Infect two hosts
    • Asexual reproduction in secondary host
    • Use secondary host to infect primary host
    • Can survive if one host is unavailable
    • Can spread through different environments (water, land)


  • Roundworms that live in the small intestine of dogs
  • Lifecycle
    • Dogs accidentally swallows worm eggs from contaminated soil
      • Eggs hatch into larvae and cross intestinal wall
      • Distributed in body tissues but remain dormant (inactive)
      • Larvae are reactivated in pregnant female dogs
      • Cross placenta or mammary glands → infect offspring
    • Larvae stay in puppies’ intestine and become mature worms
      • Produce eggs and are excreted in faeces
      • Some larvae spread to lungs, are coughed up, and swallowed back to small intestine
      • This way, puppies can infect humans by licking their face
  • In humans, larvae migrate to body organs but cannot reproduce
    • Often they only cause constitutional symptoms
    • Severe or repeated infections may cause swelling of organs or CNS
    • May enter eye and cause reduced vision or blindness
  • Adaptations
    • Sticky eggs increase infection
    • Attaches to intestine by hooks
    • Lacks unnecessary organs (locomotory, nervous system)
    • Produces large amount of eggs to increase infection
    • Anaerobic respiration allows survival in intestine
    • Produces enzymes that prevent degradation by host's digestive enzymes
    • Larvae form cysts around them which allows them to hide from immune system