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Communicable Diseases

Part of 4.3 Infection and Response.

Communicable disease is about transmission, defence and treatment. The important pattern is that pathogens enter a host, multiply, and trigger both natural and medical responses.

Learning Objectives

ID Official specification wording Main teaching sections
4.3.1-lo-1 4.3.1.1 Students should be able to explain how diseases caused by viruses, bacteria, protists and fungi are spread in animals and plants.
4.3.1.1 Students should be able to explain how the spread of diseases can be reduced or prevented.
4.3.1.1 Pathogens are microorganisms that cause infectious disease. Pathogens may be viruses, bacteria, protists or fungi. They may infect plants or animals and can be spread by direct contact, by water or by air.
4.3.1.1 Bacteria and viruses may reproduce rapidly inside the body.
4.3.1.1 Bacteria may produce poisons (toxins) that damage tissues and make feel ill.
4.3.1.1 Viruses live and reproduce inside cells, causing cell damage.		 Pathogens and Disease Examples
4.3.1-lo-2 4.3.1.2 Measles is a viral disease showing symptoms of fever and a red skin rash. Measles is a serious illness that can be fatal if complications arise.
4.3.1.2 For this reason most young children are vaccinated against measles.
4.3.1.2 The measles virus is spread by inhalation of droplets from sneezes and coughs.
4.3.1.2 HIV initially causes a flu-like illness. Unless successfully controlled with antiretroviral drugs the virus attacks the body’s immune cells. Late stage HIV infection, or AIDS, occurs when the body’s immune system becomes so badly damaged it can no longer deal with other infections or cancers. HIV is spread by sexual contact or exchange of body fluids such as blood which occurs when drug users share needles.
4.3.1.2 Tobacco mosaic virus (TMV) is a widespread plant pathogen affecting many species of plants including tomatoes. It gives a distinctive ‘mosaic’ pattern of discolouration on the leaves which affects the growth of the plant due to lack of photosynthesis.
4.3.1.3 Salmonella food poisoning is spread by bacteria ingested in food, or on food prepared in unhygienic conditions. In the UK, poultry are vaccinated against Salmonella to control the spread. Fever, abdominal cramps, vomiting and diarrhoea are caused by the bacteria and the toxins they secrete.
4.3.1.3 Gonorrhoea is a sexually transmitted disease (STD) with symptoms of a thick yellow or green discharge from the vagina or penis and pain on urinating. It is caused by a bacterium and was easily treated with the antibiotic penicillin until many resistant strains appeared. Gonorrhoea is spread by sexual contact. The spread can be controlled by treatment with antibiotics or the use of a barrier method of contraception such as condom.
4.3.1.4 Rose black spot is a fungal disease where purple or black spots develop on leaves, which often turn yellow and drop early. It affects the growth the plant as photosynthesis is reduced. It is spread in the environment by water or wind. Rose black spot can be treated by using fungicides and/or removing and destroying the affected leaves.
4.3.1.5 The pathogens that cause malaria are protists.
4.3.1.5 The malarial protist has a life cycle that includes the mosquito. Malaria causes recurrent episodes of fever and can be fatal. The spread of malaria is controlled by preventing the vectors, mosquitos, from breeding and by using mosquito nets to avoid being bitten.		 Pathogens and Disease Examples
4.3.1-lo-3 4.3.1.6 Students should be able to describe the non-specific defence systems of the human body against pathogens, including the:
4.3.1.6 • skin
4.3.1.6 • nose
4.3.1.6 • trachea and bronchi
4.3.1.6 • stomach.
4.3.1.6 Students should be able to explain the role of the immune system in the defence against disease.
4.3.1.6 If a pathogen enters the body the immune system tries to destroy the pathogen.
4.3.1.6 White blood cells help to defend against pathogens by:
4.3.1.6 • phagocytosis
4.3.1.6 • antibody production
4.3.1.6 • antitoxin production.		 Human Defence Systems and Vaccination
4.3.1-lo-4 4.3.1.7 Students should be able to explain how vaccination will prevent illness in an individual, and how the spread of pathogens can be reduced by immunising a large proportion of the population.
4.3.1.7 Vaccination involves introducing small quantities of dead or inactive forms of a pathogen into the body to stimulate the white blood cells to produce antibodies. If the same pathogen re-enters the body the white blood cells respond quickly to produce the correct antibodies, preventing infection.
4.3.1.7 Students do not need to know details of vaccination schedules and side effects associated with specific vaccines.
4.3.1.8 Content Key opportunities development Students should be able to explain the use of antibiotics WS 1.4 and other medicines in treating disease.
4.3.1.8 Antibiotics, such as penicillin, are medicines that help to cure bacterial disease by killing infective bacteria inside the body. It is important that specific bacteria should be treated by specific antibiotics.
4.3.1.8 The use of antibiotics has greatly reduced deaths from There are infectious bacterial diseases. However, the emergence of Culturing strains resistant to antibiotics is of great concern.
4.3.1.8 There are Resistant Antibiotics cannot kill viral pathogens.
4.3.1.8 Painkillers and other medicines are used to treat the symptoms of disease but do not kill pathogens.
4.3.1.8 It is difficult to develop drugs that kill viruses without also damaging the body’s tissues.		 Human Defence Systems and Vaccination, Antibiotics, Painkillers and Drug Development
4.3.1-lo-5 4.3.1.9 Students should be able to describe the process of discovery and development of potential new medicines, including preclinical and clinical testing.
4.3.1.9 Traditionally drugs were extracted from plants and microorganisms.
4.3.1.9 • The heart drug digitalis originates from foxgloves.
4.3.1.9 • The painkiller aspirin originates from willow.
4.3.1.9 • Penicillin was discovered by Alexander Fleming from the Penicillium mould.
4.3.1.9 Most new drugs are synthesised by chemists in the pharmaceutical industry. However, the starting point may still be a chemical extracted from a plant.
4.3.1.9 New medical drugs have to be tested and trialled before being used to check that they are safe and effective.
4.3.1.9 New drugs are extensively tested for toxicity, efficacy and dose.
4.3.1.9 Preclinical testing is done in a laboratory using cells, tissues and live animals.
4.3.1.9 Clinical trials use healthy volunteers and patients.
4.3.1.9 • Very low doses of the drug are given at the start of the clinical trial.
4.3.1.9 • If the drug is found to be safe, further clinical trials are carried out to find the optimum dose for the drug.
4.3.1.9 • In double blind trials, some patients are given a placebo.		 Antibiotics, Painkillers and Drug Development

Pathogens and Disease Examples

  • Pathogens include viruses, bacteria, fungi and protists. Different pathogens cause disease in different ways, but all disrupt normal body function.
  • Useful examples include measles, HIV and tobacco mosaic virus for viral disease; salmonella and gonorrhoea for bacterial disease; rose black spot for fungal disease; and malaria for protist disease.
  • Knowing the transmission route helps explain prevention. For example, sexually transmitted infections spread differently from diseases carried by water droplets or vectors.

Human Defence Systems and Vaccination

  • The skin forms a barrier, the nose and trachea use mucus and cilia, and the stomach produces acid. These are general defences that act before the specific immune response starts.
  • White blood cells defend the body by phagocytosis, by making antibodies that bind to pathogens, and by producing antitoxins that neutralise toxins.
  • Vaccination exposes the immune system to safe forms of antigens so that memory cells are produced before real infection occurs.

Antibiotics, Painkillers and Drug Development

  • Antibiotics kill bacteria or stop them reproducing, but they do not work against viruses because viruses reproduce inside body cells.
  • Painkillers reduce symptoms without killing pathogens, so feeling better does not necessarily mean the infection has been removed.
  • New drugs are tested in the laboratory, then in clinical trials, so that toxicity, effectiveness and optimum dose can be checked carefully.

Common Confusions

  • Antibiotics vs. antivirals: Antibiotics kill bacteria; antivirals inhibit viral replication. Antibiotics don't work against viruses because viruses reproduce inside cells where antibiotics can't reach them.
  • Antitoxins vs. antibodies: Antitoxins are specific proteins that neutralise bacterial toxins; antibodies are proteins that bind to antigens on pathogens. Antitoxins are one type of antibody.
  • Vaccination vs. immunity: Vaccination introduces antigens safely; immunity is the resulting protection. Vaccination creates memory cells, so the immune response is faster if real infection occurs.
  • Painkillers vs. antibiotics: Painkillers mask symptoms (you feel better) but don't kill pathogens (infection continues). Antibiotics kill pathogens. Taking painkillers without addressing the infection is dangerous.

Key Terms

  • Pathogen: a microorganism or agent that causes disease.
  • Communicable disease: a disease that can be passed from one organism to another.
  • Vaccination: introducing antigens safely so the immune system can prepare memory cells.
  • Antibody: a protein produced by white blood cells that binds to a specific antigen.
  • Antitoxin: a protein that neutralises a specific bacterial toxin.
  • Phagocytosis: the process by which white blood cells engulf and digest pathogens.
  • Antibiotic: a drug that kills bacteria or stops them reproducing.
  • Antiviral: a drug that inhibits viral replication.
  • Pathogenic: capable of causing disease.
  • Clinical trial: careful testing of a treatment on people to assess safety and effectiveness.

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