Edexcel International A Level Biology

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6.7 Pathogens: Routes of Entry

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Pathogens: Routes of Entry

  • In order for a pathogen to cause disease it must enter the body of the host
    • Body openings, e.g. the mouth, eyes, and urinary tract, provide easy access for pathogens to enter
    • Pathogens may enter directly into the bloodstream through breaks in the skin
  • Pathogens may be transmitted in a variety of ways


  • These are living organisms that carry pathogens and transmit them between hosts
  • Insects, such as flies and mosquitoes, are common vectors for diseases like malaria and yellow fever


  • Droplets from the respiratory tract will be suspended in the air when an infected person coughs, sneezes or talks
  • These droplets contain pathogens that can be inhaled by healthy people
  • The airways provide an entry point into the respiratory system of a new host and another infection occurs, e.g. flu, measles, tuberculosis


  • Pathogens can enter through the digestive system when we ingest contaminated food or drink
  • This is especially probable if food is undercooked, as heat destroys most of the pathogens 
  • These pathogens can make their way through the lining of the gut and cause disease (e.g. cholera, Salmonella poisoning)

Indirect contact

  • Inanimate objects can contain large numbers of pathogens that may be transferred between hosts
    • An infected individual may touch or cough on an object which is later touched by a healthy individual who transfers the pathogens to their mouth or nose by touching their face
  • Examples include bedding, towels, and surfaces

Direct contact

  • Pathogens that spread this way will require some part of the host, e.g. skin, body fluids, to come into direct contact with a healthy individual
  • Pathogens that spread by this route can then pass through the mucous membranes and enter the bloodstream, e.g.
    • When shaking hands with another person who then puts their hand to their nose or mouth
    • During sexual transmission
  • Examples include HIV, ebola, syphilis


  • This typically occurs when a pathogen enters the body through broken skin, providing it with a direct route into the bloodstream
  • Transmission could be through sexual contact, sharing needles during drug use, or bites or scratches from infected animals
  • Examples include hepatitis B, HIV, tetanus, and rabies

Barriers to Pathogenic Entry


  • The skin provides a physical barrier against infection
  • If the skin is damaged it leaves the exposed tissue beneath vulnerable to pathogens
  • The blood clotting mechanism of the body plays an important role in preventing pathogen entry in the case of damage to the skin
  • Blood clotting takes time, however, so a few pathogens may still enter before a clot forms

Microorganisms of the gut and skin

  • Collectively these harmless microorganisms are known as the gut or skin flora
  • They compete with pathogens for resources, thereby limiting their numbers and therefore their ability to infect the body

Stomach acid

  • The hydrochloric acid that makes up a large part of the gastric juices in the stomach creates an acidic environment that is unfavourable to many pathogens present on food and drink
  • Sometimes a few of these pathogens may survive and make their way to the intestines where they infect the gut wall cells and cause disease


  • Secretions of the mucosal surfaces, e.g. tears, saliva, and mucus, contains an enzyme called lysozyme
  • This enzyme will damage bacterial cell walls, causing them to burst, or lyse

Chemical and physical barriers 1Chemical and physical barriers 2

The body has various barriers that prevent the entry of pathogens

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Author: Marlene

Marlene graduated from Stellenbosch University, South Africa, in 2002 with a degree in Biodiversity and Ecology. After completing a PGCE (Postgraduate certificate in education) in 2003 she taught high school Biology for over 10 years at various schools across South Africa before returning to Stellenbosch University in 2014 to obtain an Honours degree in Biological Sciences. With over 16 years of teaching experience, of which the past 3 years were spent teaching IGCSE and A level Biology, Marlene is passionate about Biology and making it more approachable to her students.