Common Communicable Diseases (WJEC GCSE Biology)

Human Immunodeficiency Virus

• Human Immunodeficiency Virus (HIV) is spread by blood-to-blood contact e.g. during sexual intercourse or through the sharing of needles
• HIV destroys white blood cells in the immune system
  • Initial symptoms are flu-like
    • This includes headaches, high temperature, joint and muscle pains
  • If untreated, the virus compromises the immune system leading to the onset of Acquired Immune Deficiency Syndrome (AIDS)
    • AIDS is used to refer to several life-threatening illnesses which may result from having a compromised immune system

How HIV affects the immune system

• The virus infects a certain type of lymphocyte of the body's immune system
• Normally lymphocytes seek out and destroy pathogens that enter the body, producing antibodies that attach to pathogens
• However, HIV avoids being recognised and destroyed by lymphocytes 
• It infects lymphocytes and uses the cells’ machinery to multiply which reduces the number of lymphocytes in the immune system, and also the number of antibodies that can be made
• This decreases the body’s ability to fight off infections e.g. tuberculosis or pneumonia, eventually leading to AIDS (Acquired immunodeficiency)

How HIV affects the immune system diagram

Controlling the spread of HIV

• The spread of STIs such as HIV is best controlled by:
  • Limiting the number of sexual partners an individual has
  • Not having unprotected sex, but making sure to always use a condom
  • Getting tested if unprotected sex or sex with multiple partners has occurred
  • Raising awareness through education programmes
  • Anti-viral drugs can be used to prevent progression of the disease in infected individuals
    • This treatment will only prevent the multiplication of the virus inside cells and must be taken throughout the lifetime of the infected individual

Chlamydia

• Chlamydia is a common sexually transmitted disease caused by the bacterium Chlamydia trachmatis
• The disease is spread via the vagina and urethra during sexual intercourse
  • It can also be passed to babies during the birth process affecting their lungs and causing conjunctivitis in their eyes
• Symptoms can be mild including:
  • pain when urinating
  • unusual discharge
  • abdominal pain in women
  • testicular pain in men
• If untreated, more serious symptoms may include:
  • pelvic inflammatory disease
  • reactive arthritis
  • infertility

Controlling the spread of chlamydia

• The spread of STIs such as chlamydia is best controlled by:
  • Limiting the number of sexual partners an individual has
  • Not having unprotected sex, but making sure to always use a condom
  • Getting tested if unprotected sex or sex with multiple partners has occurred
  • Raising awareness through education programmes
  • Completing a course of antibiotics such as tetracycline or erythromycin

Malaria

• The pathogens that cause malaria are single-celled protists from the Plasmodium family (four species)
• The malarial protist has a life cycle that includes the mosquito as a vector
• Plasmodium destroys red blood cells
  • This causes recurrent episodes of fever and can be fatal in certain instances

How malaria infects a human

• Malaria protists are transmitted to humans by an insect vector:
  • Female Anopheles mosquitoes feed on human blood to obtain the protein they need to develop their eggs
  • If the person they bite is infected with Plasmodium, the mosquito will take up some of the pathogen with the blood meal
  • When feeding on the next human, Plasmodium passes from the mosquito to the new human’s blood
• Malaria may also be transmitted during blood transfusion and when unsterile needles are re-used
• Plasmodium can also pass from mother to child across the placenta

How malaria infects a human diagram

Malaria is transmitted to a human when they are bitten by a mosquito carrying the pathogen

Controlling the spread of malaria

• The different methods for reducing malaria are:
  • Reducing the number of Anopheles mosquitoes (females) in an area
    • This can be achieved by: 
      • Releasing large numbers of infertile male mosquitoes so that no further offspring can be produced
      • Spraying living areas with insecticides, such as DDT
      • Spreading oil over the surfaces of water bodies that the mosquitoes breed in such as ponds and irrigation or drainage ditches (the mosquitoes lay their eggs in water but the larvae breathe air at the water surface – an oil layer makes this impossible and kills the larvae)
      • Draining marshes and other unnecessary bodies of water
      • Biological control:
        • Ensuring ponds and irrigation or drainage ditches are stocked with fish that feed on mosquito larvae
        • Spraying these water bodies with a preparation containing the bacterium Bacillus thuringiensis, which kills mosquito larvae but is not toxic to other organisms
  • Reducing the chance of being bitten by these mosquitoes
    • This can be achieved by:
      • Sleeping under bed nets (which can also be soaked periodically in insecticide to increase effectiveness) 
      • Avoiding skin exposure at dusk when mosquitoes are most active
      • Using mosquito repellent 
  • Using drugs to prevent Plasmodium from infecting humans
    • Anti-malarial drugs include paludrine or daraprim
    • A vaccination has been developed to prevent infection