CH11 Infectious Diseases

• Disease


  A condition that threatens our health by disrupting the functions of human body systems
• Infectious disease


  • Caused by pathogens such as bacteria, fungi, parasites, or viruses
  • Can be transferred from one organism to another through bodily fluids, food, and water
• Infectious diseases


  • influenza (airborne/droplet transmission; influenza virus infection)
  • pneumococcal disease (airborne/droplet transmission; Streptococcus pneumonia bacteria infection)
  • syphilis (sexually transmitted; Treponema pallidum bacteria infection)
  • dengue fever (vector/mosquito transmitted; dengue virus infection)

  
• Non-infectious disease


  • Caused by malnutrition, genetics, physiological abnormalities, or degeneration due to old age
  • Cannot be transferred between individuals
• Non-infectious diseases


  • Coronary heart disease (caused by high fat, high salt diet coupled with stressful and sedentary lifestyle)
  • Sickle cell anaemia (caused by mutation that changed the structure of haemoglobin A (HbA) gene in chromosome 11, resulting in a variant of haemoglobin that is less soluble and does not bind readily with oxygen)
• Bacteria


  • Always has a cell surface membrane and sometimes, a (peptidoglycan) cell wall
  • Genetic material is not enclosed in a nucleus and free-floating in cytoplasm
  • Both chromosomal and plasmid genetic material is DNA
  • Free-living cells that do not require a host and can reproduce using its own enzymes and ribosomes
  • Some are pathogenic, some are not
• Virus


  • Always has a protein coat (capsid) and sometimes, a lipid envelope
  • Genetic material is enclosed in the capsid
  • Genetic material could be DNA or RNA
  • Obligate intracellular parasite requires a living host cell and uses host cell's enzymes and ribosomes to reproduce
  • All are pathogenic

  
• Pneumococcal disease


  Caused by bacteria - Streptococcus pneumoniae
• Influenza


  Caused by virus - Influenza virus

  
• Signs of pneumococcal disease and influenza


  • Cough, fever
  • Difficulty breathing / shortness of breath
  • Sore throat, runny or stuffy nose
• Symptoms of pneumococcal disease and influenza

  • Headache (pneumococcal meningitis)
  • Chest pain (pneumococcal pneumonia)
  • Ear pain (otitis media)
  • Headache
  • Muscle aches
  • Fatigue / tiredness
• Treatment for pneumococcal disease and influenza


  • Antibiotics
  • Home rest
• Modes of transmission for pneumococcal disease and influenza


  • Direct contact with respiratory secretions, containing the pathogen, like saliva or mucus
  • Transfer of airborne respiratory droplets, containing the pathogen, into mouth, nose, or eyes
• Methods to reduce transmission of pneumococcal disease and influenza


  • Getting vaccinated
  • Frequent handwashing with soap and water
  • Frequent sanitization of common-use, frequently touched surfaces
  • Wearing a mask or isolating when unwell
• Primary immune response


  1. At day 0, the body had its first exposure to a pathogen with antigen A

  2. Lymphocytes that are exposed to antigen A undergo cell differentiation and cell division to produce effector lymphocytes and memory lymphocytes

  3. Time is needed for cell differentiation and cell division, hence, a primary immune response appears only after a lag period of several days (around 8 days)

  4. Effector lymphocytes are short-lived, actively produce and secrete anti-A antibodies, and cause agglutination of pathogens and signal phagocytes to carry out phagocytosis

  5. Memory lymphocytes are long-lived, remain in the blood stream for months or even years, and are more easily and more quickly induced to become effector cells by a later encounter with the same antigen

  
• Secondary immune response


  1. At day 40, when the pathogen with antigen A infects the body again, memory lymphocytes recognize the specific antigen A and triggers a secondary immune response with shorter lag time and higher intensity (more anti-A antibodies produced and released into the bloodstream)

  2. This results in a more sensitive and rapid immune response against the pathogen resulting in reinfection with less severe signs and symptoms / immunity against this specific strain of pathogen

  
• Vaccines


  • Are made of an agent that resembles a pathogen (e.g. weakened or dead bacteria / parts of the virus protein coat)
  • Stimulate a primary immune response and the production of lymphocytes that recognise the specific antigens on the pathogen
  • Allow lymphocytes to quickly produce a large amount of antibodies when the pathogen infects the body
  • Hence prevent spread of infectious diseases

  
• Antibiotics


  • Are chemicals that block metabolic pathways of bacteria, hence bacteriostatic (preventing growth/reproduction) or bactericidal (causing bacterial cell death)
  • Inhibit enzymes involved in DNA replication, DNA transcription and RNA synthesis, protein synthesis by bacteria ribosomes, cell surface membrane functions, and bacteria cell wall synthesis
  • Ineffective against viruses due to structural and reproductive differences

  
• Development of antibiotic resistance in bacteria

  1. Random gene mutation and plasmid transfer results in genetic variation in antibiotic resistance among bacteria cells in the population

  2. Antibiotics exert a selection pressure, bacteria cells which are antibiotic resistant are better adapted because they have membrane proteins that are able to pump antibiotics out of the cell, while the bacteria cells which are sensitive to antibiotics die off from the antibiotics

  3. Antibiotic resistant bacteria survive longer and reproduce more offspring which inherits the allele for antibiotic resistance

  4. Hence, the proportion of the population having the allele for antibiotic resistance will increase gradually over many generations, leading to the evolution of the population to become antibiotic resistant