Pathogens

Body Defences

• Human body has a number of mechanisms to protect it from invading organisms

• These invaders may cause disease if body’s defences are overcome

• Body has defences against pathogens

  • External defences stop pathogens entering body

  • Internal defences **are present should invaders get past external barriers

Terminology

• Pathogen: Disease causing organism

• Communicable or infectious (transmissible) diseases: diseases caused by foreign organisms invading the body & multiplying there

• Contagious: A communicable disease that can be passed on by direct contact with a person suffering from the disease, or by contact with something touched by the person

• Vector: intermediate host of pathogen that can spread communicable disease, such as mosquitoes or fleas

• Non-communicable diseases: can be caused by environmental, genetic or other factors which don’t involve a specific pathogen. Often these “diseases” are known as disorders to distinguish them from communicable diseases

Types of Pathogens

• Bacteria

  • Most bacteria are harmless to humans (non-pathogenic)

  • Many bacteria are essential to life on Earth, through role in decomposition of organic material & cycling of elements, industrial processes. E.g. Yoghurt & cheese making

  • Huge number of bacteria live on our skin & in alimentary canal (assist in digestive process)

  • Male armpit = more than 2 MILLION bacteria per square centimetre

  • These bacteria have no ill effect on our health, whereas others can cause illness or death when present in relatively small numbers

  • Prokaryotes

  • Unicellular & can be seen only with a light microscope

  • Lack a nucleus; their DNA either floats freely in the cytoplasm or is in the form of circular plasmids

  • Classified by their cell shape

  Bacterial Infections can be treated with antibiotics

• Viruses

  • Not discovered until 1938, using electron microscope

  Viruses: An infectious agent, too small to be seen with ordinary light microscope, consisting of a protein sheath surrounding a core of nucleic acid. They are totally dependent on living cells for reproduction – cannot reproduce themselves.

  • All viruses contain genetic material in form of a molecule of either DNA or RNA, but never both DNA or RNA surrounded by a coat of protein

  • Viral infections cannot be treated with antibiotics. Anti-viral drugs can reduce symptoms of some viral illnesses.

  • When a virus infects living cell, its DNA or RNA induces the cell to manufacture more virus particles

  • New virus particles are then able to leave the host cell to infect others

  • Some viruses multiply  in bacterial cells, causing death of bacterium. Known as Bacteriophages

  • Not all viruses are harmful

  • Viruses are now used to insert new genes into other organisms

  • Bacteria that have been genetically modified in this way are used to produce insulin to treat diabetes

Structural differences between viruses and bacteria

• Protozoa and Fungi

  • Protozoa: Unicellular eukaryotic cells cause illness by ingesting infected water or through narrow nasal capillaries under high pressure. Eg. Amoebic dysentery

  • Fungi: eukaryotes with cell walls of chitin release spores. Most human fungal infections are as result of opportunity

  • Low resistance to fight infection. Eg. Candida & Athlete’s foot

• Zoonotic infections

  • Zoonotic infections: infections passed from animals to human. E.g. SARS, MERS, avian influenza (bird flu), Swine Flu (pigs), rabies, Ebola & Zika

  • Can be caused by viruses, bacteria, parasites, and fungi

  • Very common- Scientists estimate more than 6 out of every 10 infectious diseases in humans are spread from animals

Transmission of Pathogens

• Communicable disease may be spread by transmission of pathogenic organism from one person to another. Transfer can occur in a number of ways:

  • Transmission by contact

    • Transmission by contact: involves spread of pathogen by actual physical contact

    • Direct: actually touching infected person

    • Indirect: touching an object that has been touched by infected individual

    • Skin infections & sexually transmissible infections are spread by contact

  • Transfer by body fluids

    • Transfer of body fluids from one person to another results in the transmission of a number of infections

    • When blood or other body fluids from an infected person comes into contact with the mucous membranes (eg. Nose, throat, mouth, genitals) or bloodstream of an uninfected person, then pathogens may enter body of that person (such as through a needle stick or a break in the skin)

    • Human Immunodeficiency Virus, Hepatitis B & C spread in this way

  • Infection by droplets

    • Infection by droplets: may occur when tiny droplets of moisture, harbouring pathogenic organisms, are emitted when breathing, talking, sneezing or coughing.

    • Droplets may be breathed in by others, or may settle on food or utensils to be later ingested with food

    • Many viral infections, such as those causing Ebola, COVID-19, mumps, colds & influenza, may spread in this way

  • Ingestion

    • Ingestion of food or drink contaminated with pathogens may result in disease

    • i.e. Dysentery, typhoid fever, salmonella food poisoning

  • Airborne transmission

    • Airborne transmission: when moisture in exhaled droplets evaporates, many bacteria are killed, but viruses and some bacteria remain viable & can cause infection when inhaled

    • As these particles are lighter, they remain viable for a greater distance than those transmitted by droplet

    • i.e. Measles, chickenpox

  • Transmission by vectors

    • Transmission by vectors: transfer of pathogens by other animals, such as insects, ticks or mites.

    • Some vectors transfer the pathogen directly

    • Others may spread pathogen to food or water, which is then ingested (i.e. house flies)

    • Many vector-borne diseases are spread by a specific vector

      • Mosquitoes: Malaria & Dengue fever

      • Tsetse fly: Trypanosomiasis (African Sleeping sickness)

      • Ticks: Lyme Disease

      • Fleas (from rats & mice): Bubonic plague