Pathogens & Barriers (Cambridge (CIE) O Level Biology): Revision Note

Pathogens

• A pathogen is a disease-causing organism

• Pathogens are passed on from one host to another and therefore the diseases they cause are known as transmissible diseases

• Pathogens can be passed on from host to host in different ways, including:

  • Direct contact - the pathogen is passed directly from one host to another by transfer of body fluids such as blood or semen (eg HIV, gonorrhoea, hepatitis B & C)

  • Indirect contact - the pathogen leaves the host and is carried in some way to another, uninfected individual

Methods of transmission table

The Body Defences

• The body has several barriers to the entry of pathogens; these barriers can be either mechanical or chemical

  • Mechanical, or physical, barriers make it difficult for pathogens to get past them and into the body

  • Chemical barriers are substances produced by the body cells that trap or kill pathogens before they can get further into the body and cause disease

• Examples of barriers to the entry of pathogens include:

  • Skin covers almost all parts of the body to prevent infection from pathogens; if it is cut or grazed, it immediately begins to heal itself, often by forming a scab

  • Mucus and hairs in the nose and airways make it difficult for pathogens to get past them and into the airways and lungs

    • Mucus traps pathogens

    • The hairs waft the mucus out of the airways

  • The stomach contains hydrochloric acid which is strong enough to kill any pathogens that are swallowed

• Other examples include enzymes in tears, and sebum, an oily substance produced by the skin