Principles of Disease and Epidemiology

Epidemiology is the study of the frequency, distribution and causes of diseases in a given population.

Definitions:

• Infection: the growth of a pathogen in a host organism.  Infection depends on exposure to the pathogen and host susceptibility. 


• Disease: the response by a host to an infection, which when bad evokes a recognizable pattern of clinical symptoms.


• Incidence is the number of new cases divided by time.


• Incidence rate is the number of new cases divided by the total population at risk.

• Prevalence is the number of cases existing at any moment in time.


• Prevalence rat is the number of cases divided by the total population at risk.


• Etiology: study of disease causing pathogen.


• Symbiosis: interactive relationship between two species.


• Residents: colonizing microbes.


• Transients: temporary microbes.


• Opportunists: normal flora which become pathogenic under certain circumstances.


• Pathogens: disease causing microbes.


• Immuno compromised: a organism with a weakened immunity.


• Commensalism: the interaction of two organisms in which one benefits and the other is neither harmed nor helped by the interaction.


• Mutualism: relationship where both the host and microbe are metabolically dependent on each other e.g. lichen have a symbiotic association with a fungus.


• Parasitism: the interaction of two (or more) organisms where one is benefited and the other harmed by the relationship.


• Disease Severity: Acute: short duration, Chronic: lasts for a longer time period, Sub-acute: duration between acute and chronic, Latent: causative agent lies dormant within the body and suddenly become active resulting in disease.


• Reservoir: Also known as an asymptomatic infection carrier it is an organisms that carries and spreads the pathogen but is unharmed itself.

Types of Epidemiology

• Descriptive Epidemiology


• Analytical Epidemiology


• Retrospective Epidemiology


• Prospective epidemiology


• Experimental Epidemiology


• Serological Epidemiology

Application of Epidemiology

• Tracking of health problems occurring in a community.


• Establish the clinical picture of the disease or health problem in a community.


• Estimating risk of specific diseases or syndromes.


• Identify syndromes, precursors and treatments for diseases.


• Investigate epidemiology of unknown etiology.


• Establish the history of a disease in a defined population.

Koch�s Postulates & Exceptions

• Pathogen must be present in every instance of disease.


• Pathogen must be grown in pure culture.


• Pathogen must be capable of being re-isolated from an inoculated animal expressing the disease.


• Exception:  Some bacteria and viruses cannot be grown in the laboratory.


• Exception:  Some diseases are caused by several microbes.


• Exception:  Some pathogens are responsible for causing different diseases.


• Exception:  Some pathogens can not be grown in animals and exist only in humans.

Distribution Formats

• Sporadic: Few cases


• Endemic: Cases in a local region or area.


• Epidemic: Wide spread outbreak of a disease in excess of what is normally expected. disease like syndrome.


• Pandemic: Spreading throughout the globe.

Disease

• Disease is an interaction between a Host, Pathogen (agent of infection) and Environment.


• Classification of Disease: Communicable Diseases, Contagious Diseases and non-Communicable Diseases.


• Stages of Disease Development:
  • Incubation period: pathogen is multiplying.
  
  
  • Prodromal period:  symptoms being to appear
  
  
  • Illness:  visible signs of disease.
  
  
  • Period of decline: pathogen is under control
  
  
  • Period of convalescence: patient begins to recover.
  
  

Iceberg Concept of Infection

• Cell Response: Exposure no cell entry�Incomplete viral maturation�Cell transformation�Cytophatic effect�Fatal


• Host Response: Exposure but no infections�Infections with no clinical illness�Mild illness�Severe disease�Fatal


• Mechanisms of Transmission: Aerosolizing, touch, insect bite, animal bite fomites.


• Ecological factors of infections:  altered environment, changes in food production, deforestation, global warming, increased use of antibiotics and bacteria in the air.

Factors Influencing Outbreaks and Disease Spread

• Factors effecting outbreak: presence of an infected host, adequate number of potential hosts and an effective methods of transmission by contact.


• Factors affecting spread: stability of virus within it�s the environment, number of virion particles releases, virulence and invasiveness of pathogen, availability of proper vector or medium for spread. 

Normal Flora

• Can be residents or transients.  Resident population remains constant and prevents invagination by pathogens and raises overall immunity.  Residents can in rare situations become opportunists and cause infections.  Transient population number varies.


• In the body normal biota can inhabit: skin, respiratory tract, intestine, mouth, nose, throat and vagina.


• Benefits of Normal Flora include: resistance to some pathogens, release of bacteriocins and colicins, production of vitamin K, continued antigenic stimulation from commensals. 


• Disadvantages of Normal Flora:  commensal bacteria may cause localized infections, can become pathogenic if they acquire virulence factors or are introduced to sterile sties, contribute to body odor.