Infectious disease

• Infectious diseases


  Diseases caused by pathogens that are transmissible (can be spread between individuals within a population)
• Non-infectious diseases


  Long-term, degenerative diseases that are not caused by pathogens
• Four infectious diseases of worldwide importance

  • Cholera
  • Malaria
  • Tuberculosis (TB)
  • HIV/AIDS
• Worldwide the number of people infected with these diseases remains very high and these diseases pose serious public health problems now and for the foreseeable future
• To control a disease, it is very important to know what causes it (known as the causative agent or pathogen)
• Cholera


  Caused by the bacterium Vibrio cholerae, water-borne and food-borne disease
• Malaria


  Caused by four species of the protoctist Plasmodium, transmitted by female Anopheles mosquitoes
• Tuberculosis (TB)


  Caused by two species of bacteria: Mycobacterium tuberculosis and Mycobacterium bovis
• HIV/AIDS


  Caused by the Human Immunodeficiency Virus (HIV), a retrovirus
• Transmission of cholera


  1. Contaminated water and food

  2. Infected people egesting bacteria in faeces

  3. Faeces contaminating water supply or food/utensils
• Transmission of malaria


  1. Infected Anopheles mosquito takes blood meal

  2. Parasites enter human bloodstream and liver cells

  3. Parasites multiply and enter red blood cells
• Transmission of tuberculosis (TB)

  1. Infected people coughing/sneezing release bacteria in droplets

  2. Uninfected people inhale droplets
• Transmission of HIV/AIDS


  1. Exchange of body fluids during sexual intercourse

  2. Blood donation

  3. Sharing needles

  4. Mother to child across placenta, during birth, or through breastfeeding
• Cholera, malaria, TB & HIV/AIDS summary

  • Pathogen
  • Method/route of transmission
  • Site of action of pathogen
  • Signs and symptoms
• Preventing and controlling cholera

  • Providing adequate sanitation infrastructure and clean water supply
  • Vaccination programmes
  • Access to oral rehydration therapy
• Preventing and controlling malaria

  • Reducing Anopheles mosquito population
  • Using prophylactic drugs
  • Avoiding mosquito bites
  • Improving diagnosis and drug supply
• Preventing and controlling tuberculosis (TB)


  • Contact tracing and testing
  • BCG vaccination
  • Testing and destroying infected cattle
  • Pasteurising milk
• Preventing and controlling HIV/AIDS

  • Screening blood donations
  • Heat-treating blood to kill virus
  • Difficulty in developing vaccine due to virus changing surface proteins
• Tuberculosis (TB)


  Form of TB that can be transmitted between cattle and humans (caused by Mycobacterium bovis)
• Preventing the form of TB that can be transmitted between cattle and humans

  1. Routinely testing cattle for TB and destroying those that test positive

  2. Pasteurising milk (kills any TB-causing bacteria present in the milk)

  3. Ensuring meat is cooked properly
• Opportunistic disease


  A disease that takes advantage of a weakened immune system
• Preventing the spread of HIV is very difficult, as the virus has a long latent stage, which results in it being transmitted by people who have the virus but show no symptoms and do not know they are infected
• The virus can change its surface proteins, making it difficult for the human immune system to recognise it and for a vaccine to be developed
• Preventing the transmission of HIV

  1. Blood donations can be screened for HIV and heat-treated to kill any viruses

  2. HIV-positive mothers and their babies can be treated with drugs (as HIV can be transmitted across the placenta, during birth and through breast milk)

  3. Condoms, femidoms and dental dams can be used to decrease the infection risk during sexual intercourse and oral sex by forming a physical barrier between body and fluids

  4. Education programmes about how the virus is transmitted can be released into the community to encourage people to change their behaviours in order to protect themselves and others

  5. Intravenous drug users encouraged not to share needles
• Controlling HIV

  1. Contact tracing (and the subsequent testing of those contacts for the virus)

  2. Screening blood donations

  3. Public health measures widespread HIV testing of the population and education programmes

  4. Needle-exchange schemes have been set up in some places to exchange used needles for new, sterile ones

  5. Encouraging high-risk groups (eg. male homosexuals, prostitutes, injecting drug users) to be tested

  6. Using anti-retroviral drugs
• The socio-economic status of a person or country with HIV can determine how it is controlled
• Antibiotics


  Drugs that kill or stop the growth of bacteria (prokaryotes) but do not harm the cells of the infected organism
• Penicillin


  An antibiotic derived from living organisms (certain fungi in the genus Penicillium)
• How antibiotics work


  Interfering with the growth or metabolism of the target bacterium by targeting processes like synthesis of bacterial cell walls, activity of proteins in bacterial cell surface membranes, bacterial enzyme action, bacterial DNA synthesis, and bacterial protein synthesis
• How penicillin affects bacteria

  • Penicillin stops the cross-links forming in the bacterial cell wall by inhibiting the enzymes that catalyse their formation
  • The autolysins keep creating holes in the bacterial cell wall, making the walls weaker and weaker
  • The weakened cell walls eventually burst as bacteria take up water by osmosis and the pressure exerted on them from within the cell becomes too much
• Penicillin (and other antibiotics) do not affect viruses as they do not have cells (or cell walls) and therefore cannot be targeted in the ways that an antibiotic targets a bacterial cell
• Penicillin is not effective against all bacteria (eg. tuberculosis) because the bacteria may have thick cell walls which reduce permeability, enzymes which breakdown penicillin, or developed pumps to prevent their action
• Antivirals


  Drugs that target viruses
• Vaccines

  Substances that stimulate the body's immune system to produce antibodies and provide immunity against a disease
• How antibiotic resistance develops in bacteria

  1. Chance mutation causes some bacteria to become resistant

  2. When the population is treated with the antibiotic, the resistant bacteria do not die

  3. The resistant bacteria can continue to reproduce with less competition from the non-resistant bacteria

  4. The genes for antibiotic resistance are passed on with a much greater frequency to the next generation

  5. Over time, the whole population of bacteria becomes antibiotic-resistant
• Vertical transmission

  The spread of antibiotic resistance within a single bacterial population
• Horizontal transmission


  The spread of antibiotic resistance within a single bacterial population, between two populations of the same species of bacteria, or occasionally between populations of different species of bacteria
• Antibiotic resistance in bacteria is an example of natural selection that humans have helped to develop due to the overuse of antibiotics
• Commonly prescribed antibiotics are becoming less effective due to overuse of antibiotics, large scale use in farming, and patients failing to complete the full course of antibiotics
• Superbugs

  Bacteria that have developed resistance to multiple antibiotics