01. General Principles Flashcards
Why Study Infectious Disease?
- Top Ten Leading Causes of Death
- Top Ten Causes of Death in LowIncome Countries
- Kids are dying
- Preventable
- Emerging and Re-emerging
Infectious Disease Classification
- Clinicians: Clinical manifestation or organ system affected (E.g., Diarrheal disease, respiratory disease, sepsis (disseminated))
- Microbiologists: Characteristics of causative organism (E.g., Bacterial, Viral, Fungal, Parasitic)
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Epidemiologists: Epidemiologic characteristics
- Means of transmission: 5 distinct categories
- Reservoir: habitat where the agent normally lives, grows, & multiplies
Means of Transmission (5)
Reservoir
Any human, animal, plant, soil or substance in which an infectious agent normally lives and multiplies
Epidemiologic Characteristics (6)
Incubation Period
Infectivity
Pathogenicity
Virulence
Inapparent Infections
Carrier State
Epidemiologic Characteristics: Incubation Period
- Time between exposure to an infectious agent and the onset of symptoms or signs of infection
- Each infectious agent has a typical incubation period, but will show variation between individuals
- Can vary widely, depending on the agent
Epidemiologic Characteristics: Infectivity
- Ability of an agent to cause infection in a susceptible host
- An individual may be infected with an organism, but not show signs of disease
Epidemiologic Characteristics: Pathogenicity
Inherent ability of an infectious agent to induce disease (harm the host)
Result of specific host-pathogen interactions
Epidemiologic Characteristics: Virulence
Severity of disease after infection has occurred
Best measured by the case fatality rate or the proportion of cases that develop severe disease
Epidemiologic Characteristics: Inapparent Infections
- Infection that can be documented by isolation of an organism (via various laboratory methodologies) in a person that remains healthy (asymptomatic)
- Can play a role in the propagation of an epidemic
Epidemiologic Characteristics: Carrier State
- An individual with an inapparent infection
- Individuals can be important in the propagation of infectious disease that spreads from person to person
- Outbreaks have been documented due to individuals that chronically carry organisms in respiratory tract, blood, stool, and genital tract
Course of Infection
- Incubation Period: Time between exposure to a disease and the development of signs or symptoms of illness
- Latent Period: Time between exposure to a disease and becoming infectious (can transmit the disease to others)
- Infectious Period: Period of time when the infected individual is infectious (can transmit the disease to others)
Herd Immunit
- When transmission is based on contact between an infected person and susceptible person, and if the number of immune persons (due to vaccine and/or natural infection) is large enough that it is unlikely that a susceptible person will have contact with an infected person, the population is said to have herd immunity
- Susceptible individuals remain in the population, but epidemics are not sustained due to the fact that the infected person and susceptible person are not coming in contact
- Level of immunity required to attain herd immunity depends on the characteristics of the infectious disease
- Herd immunity can help protect individuals who cannot be vaccinated, e.g., infants, immunocompromised
Basic Reproductive Number (R0 )
In a completely susceptible population, this is the number of individuals that one infected person typically passes the infectious disease along to;
The higher the R0, the higher the level of immunity required to attain herd immunity in a population. (Disease that spreads more readily will require a higher level of immunity to attain herd immunity)
Epidemiologic Triangle
- a framework for organizing these determinants and emphasizes the interrelation between three components: Host / Agent / Environment;
- In this model, disease results from the interaction between the agent and the susceptible host in an environment that supports transmission of the agent from a source to that host.
