Infectious disease

• Infectious diseases are caused by organisms known as pathogens
• Some infectious diseases can only spread from one person to another by direct contact, because the pathogen cannot survive outside the human body
• Pathogen: an organism that causes disease
• Disease transmission: the transfer of a pathogen from a person infected with that pathogen to an uninfected person; transmission may occur by direct contact, through the air or water
• Disease carrier (or simply carrier): person infected with a pathogen who shows no symptoms, but can be the source of infection in other people (not carrier of an inherited disease)
• Transmission cycle: the passage of a pathogen from one host to another is continually repeated as the pathogen infects new hosts
• Disease eradication: the complete breakage of the transmission cycle of a pathogen so that there are no more cases of the disease caused by the pathogen anywhere in the world
• Endemic disease: a disease that is always in a population
• Incidence of a disease: the number of people who are diagnosed over a certain period of time, usually a week, month or year
• Prevalence of a disease: the number of people who have that disease at any one time
• Epidemic: a sudden increase in the number of people with a disease
• Pandemic: an increase in the number of cases throughout a continent or across the world
• Mortality rate: the number of deaths over a particular length of time (usually a year)
• Cholera bacteria need to pass through the stomach to reach the small intestine, where they multiply and secrete the toxin choleragen
• Choleragen disrupts the functions of the epithelium lining the intestine, causing salts and water to leave the blood and resulting in severe diarrhea
• Cholera can be treated with a solution of salts and glucose through oral rehydration therapy or intravenously if the patient cannot drink
• Glucose is absorbed into the blood and linked to the uptake of sodium and potassium ions, helping to maintain osmotic balance
• Cholera transmission is commonly caused by the use of raw human sewage to irrigate vegetables, inadequate cooking, or washing in contaminated water
• Cholera is almost unknown in the developed world due to sewage treatment, provision of clean piped water, and chlorination to kill bacteria, breaking the transmission cycle
• Types of infectious diseases


  • Common cold
  • Measles
  • Influenza
  • Tuberculosis (TB)
  • HIV/AIDS
• Infectious diseases


  • Some affect us for only a short period of time
  • Others may last a much longer time
  • Some have no cure and treatments must be taken for life
• Direct contact transmission


  Pathogen cannot survive outside the human body
• Indirect transmission


  Pathogen can survive in water, human food, faeces or animals (including insects)
• Disease carriers


  People who spread a pathogen even though they do not have the disease themselves
• Transmission cycle


  The way in which a pathogen passes from one host to another
• Control methods


  1. Attempt to break transmission cycles by removing the conditions that favour the spread of the pathogen

  2. Only possible once the cause of the disease and its method of transmission are known and understood
• Vaccination


  Makes us immune to specific pathogens so that they do not live and reproduce within us and do not spread to others, breaking the transmission cycle
• Action by governments and non-governmental organisations, coordinated by the World Health Organization (WHO), are reducing the impact of these diseases
• The WHO has campaigns designed to stop all transmission of some diseases so leading to their eradication
• • Smallpox was eradicated in 1980
  • Rinderpest, a severe disease of livestock, was eradicated in 2011
• Causative agents of four diseases

  • Plasmodium
  • Virus
  • Bacteria
  • Tuberculosis
• Plasmodium


  • Eukaryotic cell structure
• Viruses are not classified as prokaryotes
• Parasites


  All viruses are parasites, but most bacteria are not
• Endemic diseases


  • Always present in populations
  • Examples: Tuberculosis in the whole human population, Malaria in tropical and sub-tropical regions
• Infectious disease


  A disease caused by an organism such as a protoctist, bacterium or virus
• Pathogen


  An organism that causes disease
• Disease transmission


  The transfer of a pathogen from a person infected with that pathogen to an uninfected person; transmission may occur by direct contact, through the air or water or by animal vectors, such as insects
• Disease carrier (or simply carrier)

  Person infected with a pathogen who shows no symptoms, but can be the source of infection in other people (not carrier of an inherited disease)
• Transmission cycle


  The passage of a pathogen from one host to another is continually repeated as the pathogen infects new hosts