4.1.1 : Communicable diseases and the immune system

• What is a communicable disease?

  A disease caused by infective organisms knows as pathogens. Pathogens include bacteria, viruses, fungi and protists.

  A communicable disease can be passed from one organism to another.
• What is a vector?

  Vectors carry pathogens from one organism to another and are involved in the spread of plant and animals diseases.

  Common vectors include water and insects.
• Pathogens: Bacteria

  Bacteria are prokaryotes

  Do not have a membrane-bound nucleus or organelles

  Their basic shapes are: bacilli, cocci, vibrios, spirilla and spirochetes.
• Bacteria: What is gram staining?

  When bacterial cell walls have different structures and react differently with a process called gram staining.
• What is positive gram staining?

  Look purple-blue under a light microscope

  E.G MRSA
• What is negative gram staining?

  Appear red under a light microscope.

  E.G. E.Coli
• Why is gram staining useful?

  Because the type of cell wall affects how bacteria react to different antibiotics.
• What are antibiotics?

  A component that kills or inhibits the growth of bacteria.
• Pathogens: Viruses

  Non-living infectious agents.

  Basic structure is genetic material surrounded by protein.

  They invade living cells, DNA of virus takes over the host cells to make more viruses.

  Reproduce rapidly, develop adaptions to their host, making them very successful pathogens.

  Cause disease in every type of organism.
• Pathogens: Protists

  Eukaryotic organisms with a wide variety of feeding methods.

  They include single-celled organisms and cells grouped into colonies.

  Infect both plant and animals, use them as their host.

  May need a vector to transfer them to their hosts ; or they may enter the body directly through polluted water.
• Pathogens: Fungi

  Not a major problem in animals.

  Eukaryotic organisms that are often multicellular.

  Cannot photosynthesise

  Digest their food extracellularly before absorbing nutrients.

  Usually, fungi infections often affect plant leaves, so they stop photosynthesising and quickly kill the plant.

  When fungi reproduce they produce millions of spore which can spread rapidly.
• What is viruses mode of action?

  Take over the cell metabolism.

  The genetic material gets into the host cell and is inserted into the host DNA.

  Virus uses the host cell to make new viruses which then burst out of the cell, destroying it and spread to infect other cells.
• What is protists mode of action?

  Take over cells and break them open as the new generations emerge.

  They do not take over the genetic material of the cell.

  They digest and use the cell contents as they reproduce (e.g protists that cause malaria).
• What is the fungi mode of action?

  Digest living cells and destroy them.

  This combined with the body's response to the damages gives the symptoms of disease.
• The production of toxins.

  Bacteria produce toxins that poison or damage the host cells, causing disease.

  Some damage the host cells by breaking down the cell membrane

  Some damage or inactivate enzymes and some interfere with the host cell genetic material so the cells cannot divide.

  Toxins are a by-product of the normal functioning of the bacteria.

  Some fungi produce toxins affect the host cells and cause disease.
• What is ring rot?

  A bacterial disease of potatoes, tomatoes.

  Caused by the gram positive bacterium CLAVIBACTER MICHIGANENSIS.

  Damages leaves, tubers and fruit.

  Can destroy 80% of the crop and there is not cure.
• What is tobacco mosaic virus (TMV)?

  A virus that infects tobacco plants.

  Damages leaves, flowers and fruit.

  Stunts growth and reduces yield and can lead to an almost total crop lost

  Resistant crop strains are available but there is no cure
• What is potato blight?

  Caused by a fungus- like protoctist Phytophthora ingestans

  Hyphae penetrate host cells, destroying leaves, tubers and fruit.

  No cure but there are resistant strains.

  Careful management and chemical treatments can reduce infection risk.
• What is black Sigatoka?

  A banana disease caused by the fungus Myscosphaerella fijiensis

  Attacks and destroys the leaves.

  Hyphae and penetrate and digest the cells, turning the leaves black.

  Resistant strains are being developed (fungicide).

  There is no cure.
• What in tuberculosis (TB)?

  A bacterial disease caused by Mycobacterium tuberculosis

  Damages and destroys lung tissue and suppresses the immune system, so the body is less able to fight off other disease.

  TB is both curable (antibiotics) and preventable (improving living standards and vaccination).
• What is bacterial meningitis?

  A bacterial infection of the meninges of the brain which can spread into the rest of the body causing blood poisoning and rapid death.

  Causes a blotchy red/purple rash that doesn't disappear.

  Antibiotics will cure the disease if delivered early.

  Vaccine can protect against some forms of bacterial meningitis.
• What is HIV/AIDS?

  Acquired immunodeficiency syndrome (AIDS).

  Targets T helper cells in the immune system.

  Gradually destroys the immune system so affected people are vulnerable to other infections.

  HIV is a retrovirus with RNA as its genetic material.

  Contains the enzyme, reverse transcriptase (transcribes RNA to a single strand to produce a single strand of DNA in the host).

  Virus is transmitted via bodily fluids (unprotected sex, sharing needles, contaminated blood products and during pregnancy).

  No vaccine and no cure, but anti-retroviral drugs slow the progress of the disease.
• What is influenza (flu)?

  A viral infection of ciliated epithelial cells in the gas exchange system.

  Kills the cells, leaving airways open to secondary infection.

  Can be fatal to young children, the elderly or people with chronic illnesses.

  Three main strains: A, B and C.

  Flu viruses mutate regularly, change is usually quite small. But there can be major changes in the surface antigens, causing pandemics as no antibodies are available.

  Vaccines can be given every year but there is no cure.
• What is malaria?

  Caused by the protist Plasmodium and spread by bites from a infected mosquito (the vector).

  Reproduces inside the female mosquito.

  Invades the red blood cells, liver and brain.

  No vaccine against it, but there are preventive measures : control the vector.

  Insecticide and removal of standing water (where they bread).

  Mosquito nets, window and door screens, long sleeved clothing can prevent the spread.
• What is ring worm?

  Fungal disease affecting animals.

  Different fungi infect different species.

  Usually caused by Trichophyton verrucosum.

  Causes grey-white, crusty, infectious and circular areas of skin.

  Not damaging but may be itchy.

  Anti fungal creams are an effective cure.
• What is athletes foot?

  A human fungal infection caused by Tina pedia (a form of ringworm that grows on and digests the warm moist skin between toes.

  Causes cracking and scaling, which is itchy and sore.

  Antifungal creams are an effective cure
• What is direct transmission?

  Pathogen is transferred directly from one individual to another by direct contact
• Direct transmission: How is a pathogen transmitted through direct contact?

  Contact with any bodily fluids of another person, e.g bacterial meningitis and sexually transmitted diseases.

  Direct skin-to-skin contact, e.g ring worm, athletes foot.

  Micro-organism from faeces transmitted on the hands, e.g diarrhoeal diseases.
• Direct transmission: How is a pathogen transmitted through inoculation?

  Through a break in the skin, e.g during sex (HIV/AIDS).

  Through an animal bite , e.g rabies.

  Through a puncture wound or through sharing needles.
• Direct transmission: How is a pathogen transmitted through ingestion?

  Taking in contaminated food or drink, or transferring pathogen from hands to the mouth, e.g diarrhoeal diseases.
• What is indirect transmission?


  Pathogen travels from one individual to another indirectly.
• Indirect transmission: How is a pathogen transmitted through fomites?

  Inanimate objects (clothing or cosmetics) that can transfer pathogens, e.g athletes foot.
• Indirect transmission: How can pathogens be transmitted through droplet infection?

  Droplets of saliva and mucus are expelled from the mouth during talking, coughing and sneezing.

  If these droplets contain pathogens, if a healthy individual breathes in the droplets they may become infected, e.g influenza or tuberculosis.
• Indirect transmission: How is a pathogen transmitted through a vector?


  A vector transmits pathogens from one host to another.

  Vectors are often animals, e.g mosquitoes transmit malaria.

  Water can also be a vector of disease, e.g diarrhoeal diseases.
• What factors can affect the transmission of communicable diseases in animals?

  Overcrowded living and working conditions.

  Poor nutrition.

  A compromised immune system (having HIV/AIDS or after surgery).

  Climate change - Introduce new vectors and new diseases.

  Culture and infrastructure.

  Socioeconomic factors - lack of trained health workers.
• How is a plant disease transmitted through direct transmission?

  Direct contact of a healthy plant with any part of a diseased plant.

  E.g ring rot, TMV.
• Indirect transmission in plants: How is a pathogen transmitted through soil contamination?


  Infected plants leaves pathogens or reproductive spores in the soil.

  These can infect the next crop.

  E.G. black Sigatoka spores, ring rot.
• Indirect transmission in plants: How is a pathogen transmitted through vectors?


  Wind - Bacteria, spores or viruses can be carried on the wind, e.g black Sigatoka.

  Water - Spores swim in the surface film of water on leaves, raindrop splashes carry pathogens and spores, e.g potato blight.

  Animals - Insects and birds can carry pathogens and spores from one plant to another as they feed.

  Humans - On hands or clothing, e.g TMV.
• What factors affect the transmission of communicable diseases in plants?

  Planting varieties of crops that are susceptible to disease.

  Overcrowding - increases likelihood of contact.

  Poor mineral nutrition - Reduces resistance.

  Damp, warm conditions - Increase survival and spread of pathogens.

  Climate change - Promote spread of diseases.
• How do plants recognise a pathogen attack?

  Plants are not passive, they respond rapidly to pathogen attacks.

  Receptors respond to molecules from the pathogens, or to chemicals produced when the plant cell wall is attacked.

  Stimulates the release of signalling molecules that switch on genes in the nucleus.

  Triggers cellular responses, e.g defensive chemicals, sending alarm signals to unaffected cells, physically strengthening cells walls.