B3 - Communicable Diseases

  • Created by: mevan
  • Created on: 25-04-21 12:25


communicable diseases can be passed on, while non communicable diseases cannot.

infectious diseases are caused by microorganisms, but not all microbes cause disease. microbes that can cause disease are called pathogens. there are four types of microbe that can cause disease: bacteria, viruses, fungi and protists.

infectious diseases can be transmitted in three ways: airborne, water and direct contact. the spread of infectious diseases can be reduced by hygene measures, isolation/quarentine and vaccination.

bacteria and viruses spread very quickly inside the body. they do this by a special form of cell division, in which each cell copies its DNA then splits to form two identical cells. 

bacteria makes us unwell by producing toxins. viruses make us unwell by reproducing inside cells, causing cell damage which eventually leads to cell lysis (where the cell bursts open).

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Viral Disease Examples


  • spread by inhalation of droplets released by coughs or sneezes
  • some symptoms of measles are red skin rash, fever, and it can be fatal if left untreated
  • it can be prevented by vaccinating young children. 


  • spread by direct contact
  • it is a flu-like illness, can lead to aids when the immune system can no longer deal with other infections or cancers
  • it is treated with anti-retroviral drugs to control the virus.

tobacco mosaic virus

  • virus only in plants, transferred between plants on gardener's hands, and enters through wounds
  • symptoms: mosaic pattern of discolouration on the leaves, affects growth due to the reduced photosynthesis
  • prevented by removing the affected plant material, watching hands and sterilising tools.
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Bacterial Disease examples


  • caused by bacteria
  • spread by eating contaminated food or preparing food in unhygienic conditions
  • symptoms are a fever, abdominal cramps, vomiting and diarrhea.
  • methods of prevention: vaccinating chickens and preparing food in hygienic conditions and cooking it thoroughly.


  • caused by bacteria
  • it is spread through direct contact via sexual contact
  • symptoms are thick yellow or green discharge from the vagina or penis, and pain in urination
  • treated with antibiotics such as penicillin (although some strains are resistant)
  • prevented by using protection in contraception such as condoms
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Fungal Disease Examples

rose black spot

  • a plant disease caused by a fungus
  • spread by wind or water
  • identified by purple or black spots on leaves that then turn yellow and drop, and poor growth due to reduced photosynthesis
  • treatments include fungicides, and removing and destroying affected leaves by burning.
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Protist Disease Examples


  • malaria is a protist
  • it is spread via mosquitos that act as vectors when they bite to feed on blood
  • sympoms are recurrent episodes of fever and in some cases death
  • can be prevented by preventing mosquitos from breeding and using mosquito nets to reduce bites.
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non-specific human defences

non-specific defences prevent microbes from entering the body. this stops all microbes, not just pathogens.

examples of non-specific defences:

  • skin - skin cells form a barrier to stop microbes from entering.
  • nose - hairs trap particles like dust that may contain microbes.
  • trachea and bronchi (airway) - cells in the trachea and bronchi produce mucus which helps trap microbes.
  • stomach - contains hydrochloric acid which kills microbes.
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the immune system - human defences

the immune system is a body system that recognises pathogens and destroys them. the most important cells in the immune system are white blood cells; they recognise microorganisms that shouldn't be inside our body and destroys them.

white blood cells have three jobs:

  • engulfing pathogens and destroying them in a process called phagocytosis (eat/kill).
  • after the bacteria produces toxins, white blood cells produce anti-toxins that counteract the toxins and stop them from working.
  • producing specific antibodies that attach to the pathogen's specific antigens, which destroys it.
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vaccination - basics

a vaccination protects against infectious diseases because it makes us immune to the disease. this means that the white blood cells are able to produce the correct antibody so quickly that the pathogen is destroyed before you become unwell. 

when you are vaccinated:

  • a dead or inactive version is injected into the body.
  • the white blood cells produce the correct antibody, which destroys the antibody.
  • if you are infected with the same pathogen again, the white blood cells can recognise it and create the correct antibodies so fast that the pathogen is destroyed before you become unwell.
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vaccination - advantages and disadvantages

advantages of vaccinating many people against a disease are:

  • reduces the spread of the pathogen because many people are immune.
  • leads to herd immunity.

some problems with vaccination are:

  • each pathogen requires a specific vaccination.
  • some pathogens do not have an effective vaccine.
  • some people cannot be vaccinated due to allergies or illness, which reduces the effect of herd immunity.
  • some people choose not to be vaccinated, which reduces the effect of herd immunity.
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treating infectious diseases


painkillers work by reducing the sensation of pain. they temporarily treat the symptoms but do not treat the cause. some examples are paracetamol, ibuprofen and asprin.


antibiotics work by killing bacteria, however they do not kill viruses. an example is penicillin - the first antibiotic to be discovered.

there are three main problems with antibiotics:

  • they do not kill viruses because they live inside cells, so it is difficult to make drugs that can get inside our cells
  • it can be difficult to identify which antibiotic to use because different ones are needed for different infections
  • antibiotics can mutate meaning that antibiotics do not work against them.
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drug development

a drug is something that alters the chemical processes in the body.


  • it is an antibiotic, and is used to kill bacteria
  • its source is the penicillium fungus (mould) discovered by alexander flemming.


  • used to treat heart conditions
  • comes from the foxglove plant


  • painkiller
  • its source is the willow tree
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testing new drugs

all drugs must be tested before they can be used.

- toxicity - is it safe?

- efficacy - is it effective?

- dose - what is the optimum dose?

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drug trials

1) drug discovery

drug is designed in a lab or extracted from a plant/microbe.

2) pre clinical trial

the drug is tested on cells, tissues and animals in a laboratory.

3) clinical trial

  • the drug is tested on a small group of humans to begin with, then used by a larger group if it is safe. 
  • a placebo containing no active ingredients is used to compare the effect of the drug with the effect of no drug.
  • in the larger group, there is a blind trial (the doctor knows who has the placebo and who does not) or a double blind trial (neither party knows who has the placebo)

4) peer review

an independent scientist checks the data which avoids bias. the drug can then be prescribed!!!!

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