1. Epidermiology And The Spread Of Disease

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Epidemiology

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Study of when and where diseases occur and how they are spread

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___ of all deaths are due to infectious diseases

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30%

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Incidence of disease

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Is the fraction of a population that contracts (newly acquires) the disease during a specific time period
-Incidence of disease
is number of newly reported cases per given time period/ 100,000 people at risk

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What is prevalence of disease?

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Is fraction of a population having symptoms of the disease during a specific time period (includes both newly acquired and established cases)

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Sporadic Disease

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Occurs ocassionally
-Recorded as invidual cases in geographically seaprated areas, implying that they are cases that are not related

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Endemic Disease

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Occurs continually at a relatively low level
-Pathogen that causes the disease may not be very virulent, or the majority of the individuals in a given area may be immune to the pathogen

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Epidemic Disease

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Occurs sporadically at an elevated level and occurs continuously at a low level between these sporadic outbreaks

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Pandemic disease

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Widely Distributed epidemic

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Disease outbreak

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Is when the number of disease cases increases in an area which previously had experienced only sporadic cases of the disease

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Reservoirs

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Of infection are sites in which viable infectious agents remain and from which infection of individuals may occur

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How can you describe human reservoirs?

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1) Symptomatic - currently have the disease
2) Convalescent - recovering from the disease and still harbour the infectious agent
3) Carriers; have no disease symptoms but carry the pathogen

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Describe in detail carriers

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-Carriers may be in the incubation period of the disease which precedes the development of symptoms
-These would be known as acute carriers because their carrier state is transient
-Chronic carriers are individuals who have recovered from the clinical disease but still harbour the infectious agent, or they may be individuals that carry the pathogen as part of their normal flora without the occurrence of disease

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Animal reservoirs

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-Some diseases occur in both humans and animals
-Diseases that occur primarily in wild or domestic animals but can be transmitted to humans are called zoonoses

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Insect Reservoirs

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Many diseases are transmitted by insects

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Inaminate or non living reservoirs

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Some diseases are present and are transmitted from water or soil

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What factors is the transmission of disease

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Geographical
Seasonal
Age group incidence with possible modes of transmission

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Disease limited to a certain geographical area, what does this suggest?

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Possible vector is involved

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How are respiratory pathogens spread?

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Air borne

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How are GI pathogens spread

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Food and water

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What are factors of disease spreading?

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1) Contact transmission - disease is transmitted by direct human contact
2) Common vehicle transmission - disease is transmitted by inanimate objects (formites), food, water , blood, drugs
3) Airborne transmission - disease is transmitted by droplets or dust
4) Vector transmission - disease is transmitted by living agents such as insects, ticks, mites, fleas, rodents

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Virulence

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Relative ability of a pathogen to cause disease and can be estimated by experimental studies to determine the infectious and lethal doses

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Infectious dose

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Number of bacteria required to produce disease.

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Lethal dose

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Number of bacterial required to kill
-Usually expressed as number of bacterial required to kill 50% of the population

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How does the route of entry of the pathogen into the body affect the dose?

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S. typhimurium injected intravenously into mice has LD50 of 10
S. typhimurium given orally to mice has LD50 of 5 million

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Nosocomial infections, why?

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Infections that are acquired by patients as a consequence of hospitalization

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What factors can nosocomial infections result from?

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1) Hospital treat patients with infectious disease and these patients may be reservoirs of pathogenic organisms
2) Patients are exposed to numerous pathogens in a hospital setting, many of which may be resistant to antibiotics
3) Patients often have weakened immune system( they are a compromised host) which make them more susceptibile to infection
4) Pathogens can be transferred from patient to patient by hospital staff or visitors. Often these drug resistant pathogens can be found as the normal flora of the hospital staff

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What can nosomial infections be?

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Endogenous or exogenous, or latrogenic

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Endogenous

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Caused by normal flora

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Exogenous

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Transmitted from another indiividual

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Latrogenic

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Doctor induced (from surgery, invasive diagnostic procedures, or medical implant devices)