12 - Communicable Diseases

Types of pathogens

• bacteria

• viruses

• fungi

• protoctista

Bacteria

• small proportion of bacteria are pathogens so cause communicable diseases

• prokaryotes

• can be classified by their basic shapes or by their cell walls

  • gram positive bacteria (purple-blue)

  • gram negative bacteria (red)

  • cell wall affects how it responds to antibiotics

Viruses

• smaller than bacteria

• basic structure is genetic material surrounded by protein

• invade living cells

• the genetic material of the virus takes over the biochemistry of the host cell

• reproduce rapidly

• develop adaptations to their host

• pathogenic

Protoctista

• small percentage act as pathogens

• cause communicable diseases in both animals and plants

• parasitic

• need a vector to transfer them

Fungi

• can’t photosynthesise

• digest food extracellularly before absorbing the nutrients

• saprophytes (feed on dead and decaying matter)

• when reproducing, they produce millions of tiny spores which spread across huge distances

Pathogens damaging the host tissues directly

• viruses take over the cell metabolism

  • genetic material is inserted into the host DNA

  • virus uses the host cell to make new viruses

  • the cell bursts and the viruses spread to infect other cells

• protoctista take over cells and break them open

  • digest and use cell contents as they reproduce

• fungi digest living cells and destroy them

Pathogens producing toxins which damage host tissues

• bacteria produce toxins that poison or damage the host cells

  • causes disease

  • break down cell membranes, damage or inactivate enzymes or interfere with the genetic material to prevent mitosis

• fungi produce toxins that affect host cells and cause disease

Plant diseases

• bacterial - ring rot

• virus - tobacco mosaic virus

• protoctist - potato/tomato blight

• fungi - black sigatoka

Animal diseases

• bacterial - tuberculosis & bacterial meningitis

• virus - HIV & flu

• fungi - ring worm & athlete’s foot

Direct transmission of pathogens

• direct contact (contagious disease)

  • microorganisms or bacteria transmitted through kissing, contact of bodily fluids, direct skin-to-skin contact

• inoculation

  • through a break in the skin

  • from an animal bite

  • through a puncture wound / sharing needles

• ingestion

  • contaminated food or drink

  • transferring pathogens to the mouth from hands

Indirect transmission of pathogens

• fomites

  • inanimate objects such as bedding, socks or cosmetics

  • transfer pathogens (mainly fungal)

• droplet infection

  • minute droplets of saliva/mucus are expelled from the mouth and may contain pathogens that others breathe in

• vectors

  • transmits communicable pathogens from one host to another

  • often animals (mosquitoes, fleas, dogs, foxes, and bats)

  • water can also act as a vector