Micro22 Principles of Disease and Epidemiology

Infection: state where host harbors microbes that survive and multiply in or on body tissues

Disease: process or event that results in change from the general state of good health

Symbiosis (state of living together between normal flora and host):

• Microbial antagonism: where flora competes for nutrients and spaces by producing toxic substances to invading pathogens

• Opportunistic microorganisms vs true pathogen:

  • Opportunistic microorganisms

    • Requires special opportunity to produce disease such as injury or impared host defenses

    • Same and common (frequently encountered) organism can produce variety of diseases and secondary infection

  • True pathogens

    • Able to infect healthy host under its own power and create disease

    • Each produce its own specific disease

    • Not so commonly encountered and cause primary infection, which can lead to secondary infection

Koch’s Postulate (Rules of disease): proves that certain organism causes a certain disease although with some exceptions such as it is not completely foolproof and cannot always guaranteed the results are correct

1. Organism must be present in every case of the disease

2. Organism must be isolated in pure culture from the infect host

3. Pure culture must produce the disease when inoculated (grow within) into a healthy, susceptible host

4. Organism must be re-isolated in pure culture from the experimentally injected host

Classifying Infectious Diseases:

Symptom: change in body function felt by the patient as a result of a disease

Sign: change due to a disease that can be observed and measure

Syndrome: a specific group of signs/symptoms that accompany a disease

Communicable disease: disease spread from one host to another

Noncommunicable disease: disease not spread from one host to another

Frequency categories of diseases:

1. Sporadic: isolated cases

2. Endemic: constantly present in a population

3. Epidemic: many people in a given area for a short period

4. Pandemic: worldwide

Duration of disease:

• Acute: develops rapidly but lasts for a short time

• Chronic: develops more slowly but lasts for a long time

• Subacute: between acute and chronic

• Latent: remains inactive for period of time then becomes active

Extend of disease infection:

• Local: confined to one area of the body

• Systemic: throughout the entire body ie. bacteremia, viremia, septicemia, toxemia, sepsis

• Focal: via blood vessels and spread to specific parts of the body and stay there

Development of Disease:

1. Incubation period: time between infection and appearance of first symptoms, long time

2. Prodromal period: nonspecific symptoms, short time

   1. Contagious period: most contagious time is the last half of incubation or prodromal period

3. Period of illness: when disease is most severe, appearance of specific symptoms

4. Period of decline: signs and symptoms subside, vulnerable to secondary infection

5. Period of convalescence: period of recovery from illness state

Spread of infection:

Through reservoirs such as humans, animals (zoonoses), and nonliving (soil, water, food)

Three principle routes of transmission of disease:

• Contact transmission: direct person to person, indirect through fomites (contaminated objects), or droplet infection

• Vehicle transmission: common vehicle, or airborne

• Vectors: organism that carries the pathogen such as arthropod through mechanical or biological transmission

Epidemiology:

Morbidity: incidence of a specific notifiable disease

Mortality: deaths from notifiable diseases

Morbidity rate: number of people affected in relation to the population

Mortality rate: number of deaths from a disease in relation to the population