4.1.1 Pathogens and disease

• The 4 types of pathogen are bacteria, fungi, protoctista and viruses
• An infectious disease is a disease resulting from infection of a host organism by a pathogen, which is a disease-causing organism.
• Bacteria are prokaryotes. This means they do not have a membrane-bound nucleus or organelles.
• Bacteria can be spherical, rod-shaped or corkscrew in shape.
• Bacteria reproduce by binary fission
• Viruses are regarded as non-living.
• Viruses that attack bacteria are known as bacteriophages.
• Viruses are unable to replicate outside a host cell as they require the host cell machinery (DNA, ribosomes) for protein synthesis as they do not have their own.
• The virus attaches to a specific host cell. The genetic material from the virus is injected into the host cell. The viral genes cause the host cell to make more viruses. The cell bursts open, releasing the new viruses
• Protoctista are eukaryotic organisms that include single-celled organisms and cells that live in colonies.
• A small group of Protoctista act as pathogens. They are parasites which means they use the host organism for their benefit.
• Protoctista often require a vector but can also enter the body directly e.g. via water
• Fungus can live in the skin of an animal, and its hyphae, which form a mycelium, grow under the skin surface.
• Fungal reproductive hyphae grow to the surface of the skin to release spores, causing redness and irritation.
• Fungal diseases are not a major problem in animals but can cause devastation in plants.
• Fungi are eukaryotic organisms – often multicellular, although yeasts which can cause disease such as thrush are single-celled.
• Fungi cannot photosynthesise. They must digest their food extracellularly before absorbing nutrients. They are saprophytes which means they feed on dead and decaying matter.
• Pathogens cause disease by damaging host cells or tissues directly or producing toxins which damage host tissues
• Exotoxins are produced and secreted by a normal pathogen
• Endotoxins are produced when a pathogen is damaged
• Tuberculosis (TB) is a bacterial disease affecting humans and cattle.
• Ring Rot is a bacterial disease of potatoes and tomatoes.
• HIV is a virus that affects humans, leading to AIDS
• Influenza is a virus that affects human's cilia
• Tobacco mosaic virus (TMV) is a virus that infects tobacco plants and many other species and causes leaf mottling
• Black sigatoka is a disease affecting bananas caused by a fungus which attacks and destroys the leaves.
• Athletes foot is a fungal infection affecting humans.
• Potato/Tomato late blight is caused by a protoctist
• Malaria is caused by the protoctist Plasmodium and spread by the bites of infected female Anopheles mosquitos (vector).
• We don't have malaria in the UK because the UK is too cold for the mosquito that carries the pathogen and many pathogens are damaged or killed by cold winter weather or cannot reproduce. Therefore, they tend to be more common in warmer climates.
• Direct transmission is when the pathogen/disease is transmitted directly from one organism to another.
• Direct transmission includes: physical contact between individuals, e.g., sexual intercourse, touching an infected individual, droplet infection e.g. coughing or sneezing tiny droplets of mucus or saliva directly onto someone, inoculation which is a break in the skin, e.g., sharing needles, animal bite and ingestion e.g. consuming contaminated food or drink
• The risk of direct transmission can be reduced by washing hands regularly, keeping surfaces clean, cleaning and disinfecting cuts, sterilising surgical instruments and using condoms
• Indirect transmission is when a disease is transmitted from one organism to another via an intermediate.
• Fomites and vectors are intermediates
• Fomites are inanimate objects can harbour and transfer pathogens: cosmetics, bedding, clothing.
• Vectors: an organism that spreads disease by carrying pathogens from one host to another but is unaffected
• Living conditions affect transmission by overcrowded living and working conditions increase transmission, poor nutrition and poor disposal of waste which provides breeding ground for vectors (humans)
• Social factors such as culture and infrastructure (in some countries, traditional medical practises can increase transmission) and socioeconomic factors, for example, a lack of trained health workers,

  insufficient education of the public can affect transmission
• Climate change affects transmission as new vectors and diseases can emerge; spread of malaria can be promoted as temperatures rise