COMMUNICABLE DISEASES
CHAPTER 12
Pathogens Bacteria
•Bacteria, viruses, fungi and protoctists • Prokaryotes - no membrane bound nucleus or organelles
• Shapes: rod, cocci, vibrios, spirilla
•Cause of communicable diseases
• Cell walls: gram staining - gram + = purple/blue
•Vectors: carry pathogens from one Gram - = red - useful as it shows what antibiotics to use (thickness of
organism to another eg water and peptidoglycan)
insects • Do not require host, larger than viruses, bacteria have some organelles
eg ribosomes, plasmids, flagellum and pili
• Free/ naked DNA
Bacterial diseases
• TB - infects phagocytes in lungs, weakens immune system,
lie dormant until immune system is suppressed then
becomes active to destroy lung tissue causing breathing
problems and coughing
• Meningitis - infection of the meninges of the brain
and spreads to rest of body causing septicaemia. -
noticeable red blotchy rash
Viruses Viral diseases
• Non living • HIV - targets t helper cells of immune system, retrovirus,
• 50x smaller than bacterium destroys immune system so body is susceptible to other
diseases, can take anti Retroviral drugs to slow down
• Genetic material + protein coat • Influenza - infection of the ciliated epithelium of the
• Invade living cells- genetic material takes over to reproduce airways. Kills them leaving airways open to infection. 3
main strands a, b and c. Vaccine is available.
• Reproduce rapidly + evolve quickly to survive
• Viruses which attack bacteria = bacteriophage
• “Ultimate parasites” Protoctist diseases
• Retrovirus - RNA • Malaria - caused by plasmodium,
spread by mosquitoes, reproduce in
female mosquito. Invades RBCs, liver
and the brain
• People with sickle cell anaemia are
immune
Protoctista Fungi
• Eukaryotes •Eukaryotes
• Single celled or multi celled •Often multi cellular
• Parasitic - use animals or humans as host •Cannot photosynthesise
•Saprophytes
•Spread by spores - “hyphae”
, Fungal diseases Modes of action
• Ring worm - grey crusty areas of skin - anti fungal •Virus - take over cell metabolism, inserts DNA into host cell
cream can treat it
to reproduce
• Athletes foot - digests the skin between the toes -
•Protoctists - digest and use the cell contents as they
anti fungal cream
reproduce
•Fungi - digest living cells and destroy them, some make toxins
Transmission •Bacteria - produce toxins which break down cell
•2 types: direct or indirect membranes, destroy enzymes or interfere with DNA so the
cells can’t divide. Toxins are by-product of normal function
Direct Transmission Indirect Transmission
•Direct contact - kissing or any contact •Fomites - bedding, socks (athletes foot, gangrene)
with body fluids, direct skin to skin •Droplets / inhalation - saliva or mucus (flu, TB)
contact (athletes foot, meningitis) •Vectors - passes on disease but not infected itself (cholera,
•Inoculation - break in skin, animal malaria, plague)
bite, needles (rabies, HIV,
septicaemia)
Transmission between animals + humans
•Ingestion - contaminated food or
•Bird flu (sheep to people)
drink (salmonella, diarrhoea)
•Foot and mouth (people are vector)
Factors affecting transmission in animals Factors affecting transmission in plants
•Overcrowded living •Overcrowding
•Compromised immune system •Poor mineral nutrition
•Poor disposal of waste •Damp, warm conditions
•Climate change •Susceptible species
•Culture + poor nutrition •Increased rainfall and wind
Plant transmission Preventing spread in plants
•Direct: ring rot, TMV, tomato blight and black •Leave room between plants
Sigatoka •Keep diseased and healthy apart
•Indirect: soil contamination - spores, vectors - •Clear fields
wind, water, animals, humans •Rotate crops
•Insecticides
•Biological control
Plant diseases
•Ring Rot: bacterial disease, damages leaves and
Plant defences • When plant tissue is
tubers diseased, they sacrifice
•TMV: virus spread by aphids against pathogens and replace it
• Casparian strip • This is possible because
•Potato Blight: protoctist - air borne • Waxy cuticle plants are always growing
•Black Sigatoka: fungus - attacks and destroys • Thorns/needles (meristems)
• Tannin
leaves • Poison
• Warning colours
• Bark
Plant response to pathogens • Cellulose
•Rapid
•Molecules on pathogen or chemicals released
Physical response
by infected cells are detected by receptors • Callose - synthesised within mins of attack and positions
•This triggers signalling molecules to be released between plasma membrane and cell walls of cells
which then switch on genes in the nucleus • This prevents plant cells surrounding the infection from
being infected
•These genes provide code to produce chemicals
• Lignin also made to strengthen the barrier
to defend cells, warn other cells and physically • Callose blocks sieve plates in phloem, reducing the spread
strengthen cell walls of disease
• Blocks plasmodesmata to block cytoplasm to cytoplasm
transport
