Communicable diseases

3.1.1 Communicable (infectious) diseases

Pathogens	Microorganisms that cause communicable diseases Eg bacteria, viruses, protists, fungi
How cause?	Bacteria	Produce toxins – damage tissues – ill Reproduce rapidly in binary fusion
	Viruses	Live & reproduce rapidly inside cell – cell damage
How spread?	By air	Droplet infection Ppl sneeze – expel droplet with pathogens from breathing system – other ppl breath in
	By water	Dirty water – cholera
	Direct contact	Cuts – give excess to blood eg HIV Shake hands
How prevent?	Wash hands Drink clean water Good hygiene Isolate infected individuals Destroy / control vectors eg mosquitoes Vaccination

What is pandemic? (1)

• A disease affecting ppl in many countries

3.1.2 Viral diseases

Measles

Spread by	Inhalation of droplets from coughs/sneezes
Symptoms	Fever, red skin rash (serious – blindness, brain damage, fatal)
Prevention	Vaccination Isolate infected individuals

HIV/AIDs

Spread by	Exchange of body fluids eg blood when share needles Unprotected sexual contact with infected person
Symptoms	Flu-like illness Virus attack & remain hidden in immune system AIDs (last stage of HIV) occurs when immune system becomes so badly damaged – X deal with other infections
Treatment	Antiretroviral drugs to control attack
Prevention	Use condoms Don’t share needles Use HIV +ve mother bottle to feed children

Tobacco mosaic virus (TMV)

Spread by	Contact between healthy & infected plants & vectors
Symptoms	Give distinctive ‘mosaic’ pattern of discoloration on leaves coz viruses destroy cells
How affect growth of plants?	Less chlorophyll – less photosynthesis – less glucose A) Less respiration – less energy released for growth B) Less amino acids/cellulose for growth
Prevention	Good field hygiene Good pest control Wash tools

All viral diseases – no treatment

3.1.3 Bacterial diseases

Salmonella food poisoning

Caused by	Food prepared in unhygienic condition – food contaminated with salmonella from raw meat Eat uncooked food – bacteria not killed by heat
Spread by	Bacteria ingested in food – disrupt balance of natural gut bacteria
Symptoms	Fever, abdominal cramps, vomiting & diarrhoea 8-72hrs
Prevention	Vaccination Cook meat thoroughly Keep raw meat away from cooked food Avoid washing raw meat – spread bacteria around kitchen

Gonorrhoea – sexually transmitted disease (STD)

Spread by	Unprotected sexual contact with infected person
Symptoms	Thick yellow / green discharge from vagina / penis Pain on urinating
Prevention	Use condom Reduce no of sexual partners Antibiotics
Treatment	Antibiotic penicillin

3.1.4 Fungal diseases

Rose black spot

Spread	In environment by wind & water
Symptoms	Purple / black spots develop on leaves Weaken plant & affect growth coz reduce area available for photosynthesis – turns yellow & drop early
Treatment	Use fungicides
Prevention	Remove / burn affected leaves / stems

3.1.5 Protist diseases

Malaria

Spread by	Mosquitos (act as vectors coz they transmit disease)
Symptoms	Recurrent episode of fever Fatal Shaking when protists burst out of blood cells Headaches, vomiting, diarrhoea
Treatment	Take antimalarial drugs – kill parasites in blood
Prevention	Use mosquito nets – avoid being bitten Insecticides – kill mosquitoes Remove standing water – prevent mosquitoes breeding

3.1.6 Human defence systems

Non-specific defence system

Skin	Acts as barrier Produces antimicrobial secretions – kill bacteria
Nose	Hair & mucus trap & prevent pathogens entering lungs
Trachea & bronchi	Has mucus – trap pathogens Cilia waft mucus upwards to throat – shallowed
Stomach	Produce HCl – kill pathogens in food

 White blood cell

• Phagocytes (non-specific) & Lymphocytes (specific)

How?

1.  Ingest pathogens (phagocytes)

Phagocytes move towards, changes shape, attack pathogen, engulfs & digest it with enzymes

2.  Produce antibodies (protein) (made by lymphocytes)

• Have complementary shape – specific antibody for specific pathogen
• Allow binding with antigens (foreign microorganisms)
• If same pathogen re-enters body, WBC respond quickly, produce correct antibodies, prevent infection
• 
•  3. Produce antitoxins – counteract toxins

In conclusion – lead to immunity from pathogens

3.1.7 Vaccination

How prevent?

• Contain small amount of dead or inactive forms of pathogen
• Stimulate WBC to respond & produce antibodies quickly specific to pathogen in large quantities to kill pathogen
• Reduce spread of infection – ppl immune
• Prevent illness in an individual

Herd immunity

• Immunising large proportion of population
• Reduce spread of infection / pathogens

MMR vaccine – protects against measles, mumps & rubella

3.1.8 Antibiotics and painkillers

Antibiotics eg penicillin

• Cure bacterial diseases by killing infective bacteria inside body
• Damage bacterial cells – kill bacteria

X kill virus – coz viruses live inside cells & are inaccessible to antibiotic

Overuse – speed up development of resistant strains of bacteria

How bacteria become resistant?

• Mutation
• Some resistant to antibiotics – survive
• Reproduce by binary fission
• Pass gene for resistance of offspring
• Increase population of resistant strain

Why resistant strain spread?

• Ppl not immune to it
• Treatment is not effective

How to reduce resistant strain?

• Reduce use of antibiotics for mild infection
• Patient complete course of antibiotics – kill all bacteria
• Restrict agricultural use of antibiotics

Painkillers eg aspirin, paracetamol

• Treat symptoms
• Don’t kill pathogens

3.1.9 Discovery and development of drugs

Drugs – chemical that affect body chemistry

Discovery – traditionally drug extracted from plants & microorganisms

Plants	Aspirin (painkiller) from willow Digitalis (treat heart condition) from foxgloves
Microorganism	Penicillin (antibiotic) Discovered by Alexander Fleming from Penicillium mould

Now, most new drugs are synthesised by chemists in pharmaceutical industry. However, the starting point may still be a chemical extracted from a plant.

Development – testing new drugs

1. Pre-clinical trials in lab of new drugs on cells, tissues & live animals

• Test for toxicity, dosage & efficacy

      2. Clinical trials – test on healthy volunteers & patients at very low doses

• Monitor for safety & side effects
• If drug is found to be safe, further clinical trials are carried out to find the optimum dose for the drug

      3. Double-blind trial

• Placebo & drug is randomly allocated to large no of patients in groups
• Doctors & patients don’t know who has new drug or placebo so
• Data won’t be affected by knowledge
• To verify efficiency & determine correct dose

     4. Peer review of data & analysis of result

• Prevent false claims
• Check results are valid, avoid bias

Placebo

• Tablet with no drug / chemical & has no effect
• Used to compare & prove effectiveness of drugs

Placebo effect

• Ppl expect treatment to work so they feel better even though it doesn’t do anything

Why trial is reliable? Large no of ppl

Why stopped early? Sufficient information gained

Why manufacturers don’t take part? They could cheat

Repeat experiment – improve reliability

Why data is unreliable? Ppl lies

Factor similar in volunteers – age & sex