Common Infections (Edexcel GCSE Biology: Combined Science)

Infections caused by bacteria

• Mycobacterium tuberculosis causes tuberculosis (TB) in humans
  • The bacteria infect the lungs, causing a chronic cough and bloody mucus
  • It is a disease often associated with poor hygiene and sanitation
  • M. bovine in cows can also transmit to humans to cause TB

• Vibrio cholerae causes Cholera
  • Bacteria infect the intestines causing diarrhoea, vomiting and leg cramps
  • Cholera can lead to death if not treated quickly
  • It is a disease often associated with poor hygiene and sanitation

• Helicobacter pylori causes stomach ulcers
  • Infection of the stomach commonly occurring during childhood
  • H. pylori infection often gives no symptoms, but can lead to leads to pains in the abdomen, loss of appetite, bloating and nausea

• Chlamydia trachomatis causes chlamydia
  • This is a sexually transmitted infection
  • There may be no symptoms in some patients, others may have pain when they urinate, unusual discharge or bleeding after sex
  • Chlamydia can cause infertility if left untreated

Infections caused by fungi

• Chalara Ash Dieback
  • This fungal infection originated in Asia and affects ash trees of all ages with devastating effects
  • It causes dark patches on the leaves, early leaf loss and bark lesions
  • Spores travel large distances in the wind

Infections caused by protists

• Plasmodium falciparum is a protist that causes severe forms of malaria in humans
  • The parasite is spread by mosquitoes
  • It cases damage to the blood and the liver
  • Infected individuals experience fever, chills and fatigue
  • Malaria can progress with more serious symptoms eventually leading to death

Infections caused by viruses

• Ebola Virus infects and kills body cells
  • Symptoms are flu-like initially
    • This includes headaches, high temperature, joint and muscle pains

  • Serious cases will also show diarrhoea, sickness, rashes and may lead to haemorrhagic fever (fever and internal bleeding)

• Human Immunodeficiency Virus (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

More about HIV

• 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, enhancing phagocytic activity
• However, HIV avoids being recognised and destroyed by lymphocytes by repeatedly changing its protein coat
• It then infects a certain type of lymphocyte and uses the cells’ machinery to multiply
• This reduces the number of lymphocytes of the immune system, and also the number of antibodies that can be made
• This decreases the body’s ability to fight off infections, eventually leading to AIDS (Acquired immunodeficiency)

How HIV affects lymphocytes

• Diseases that are caused by pathogens that pass from one host to another are described as transmissible diseases
• Transmission can occur through several different mechanisms:
• Airborne - droplets travelling in the air (maybe as a result of wind or sneezing/coughing) lead to infection
  • e.g. Colds, influenza, tuberculosis, Chalara ash dieback
  • These types of infection in animals can be prevented by:
    • Avoiding crowded areas
    • Good hygiene generally
    • Ventilation of homes or workspaces

  • These types of infection in plants can be prevented by:
    • Preventing imports of trees from countries known to carry the disease
    • Cutting down infected trees as soon as possible and replacing with an alternative species which is unaffected

• Waterborne - pathogens that live in dirty water are transmitted to organisms that come into contact with it
  • e.g. Cholera
  • This can be prevented by avoiding poor quality water and making sure that people have access to clean water

• Oral transmission - pathogens are ingested and transmitted to the host
  • e.g. Stomach ulcers
  • This can be prevented by having access to clean water and good hygiene standards

• Body fluids and sexual transmission - Exchange of saliva, blood or semen can carry pathogens from one host to another
  • e.g. HIV, Chlamydia, Ebola
  • Generally, body fluid transmission is prevented by:
    • Isolating infected individuals
    • Cleaning and sterilising contaminated areas

  • Sexual transmission of chlamydia and HIV can be reduced by:
    • Wearing condoms
    • Screening to detect cases and particularly after unprotected sex
    • Medications
    • Drug users with HIV should also avoid sharing needles
    • Limiting the number of sexual partners
    • Raising awareness by education programmes

• Animal vectors - Animals carry pathogens between hosts
  • e.g. Malaria
  • Animal vectors can be prevented by
    • Using insect repellent
    • Mosquito nets
    • Preventing the vector from reproducing

Transmission of Common Infections Table