Epidemiology and the Spread of Disease Flashcards
What is epidemiology?
Epidemiology is the study of when and where diseases occur and how they are spread.
In most developed countries, infectious disease is no longer perceived to be a significant threat to public health because of sanitation, vaccination and antibiotics.
A few infectious diseases still remain a major public health concern (e.g., tuberculosis).
Furthermore, new infectious diseases are continuously emerging.
How many deaths worldwide are due to infectious diseases?
Worldwide, 30% of all deaths are due to infectious diseases, many of which could be prevented with scientific, medical, economical and educational solutions.
What is the incidence of disease?
The fraction of a population that contracts (i.e., newly acquires) the disease during a specific time period.
What is the prevalence of disease?
The prevalence of disease is the fraction of a population having symptoms of the disease during a specific time period (includes both newly acquired and established cases).
What is a sporadic disease?
Occurs occasionally.
They are often recorded as individual cases in geographically separated areas, implying that they are cases that are not related.
What is an endemic disease?
Occurs continually at a relatively low level.
The 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.
What is an epidemic disease?
Occurs sporadically at an elevated level and occurs continuously at a low level between these sporadic outbreaks.
What is a pandemic disease?
A widely distributed epidemic.
What is a disease outbreak?
When the number of disease cases increases in an area which previously had experienced only sporadic cases of the disease.
What are resevoirs of infection?
Sites in which viable infectious agents remain and from which infection of individuals may occur.
Humans that carry and shed the infectious agent are human resevoirs.
What three categories do they make up?
These individuals may be:
symptomatic, i.e., currently have the disease
convalescent, i.e., recovering from the disease and still harbour the infectious agent
carriers, i.e., have no disease symptoms but carry the pathogen. Carriers may be in the incubation period of the disease which precedes the development of symptoms. These individuals would be described as acute carriers because their carrier state is transient.
Some diseases occur in both humans and animals, so there are animal resevoirs of disease as well.
What are diseases that occur primarily in animals that can be transmitted to humans called?
zoonoses
What are chronic carriers?
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 microbiota without the occurrence of disease.
Discuss insect reservoirs.
Many diseases are transmitted by insects such as fleas, lice and flies.
Insects that fly also have significant epidemiological implications as they have broader home ranges than non-flying insects, thus making control more difficult.
Discuss inanimate reservoirs.
Inanimate or non-living reservoirs – some diseases are present and are transmitted from water or soil.
What is contact transmission?
Disease is transmitted by direct human contact (e.g., skin pathogens).
What is common vehicle transmission?
Disease is transmitted by inanimate objects (fomites), food, water, blood, drugs (e.g., gastrointestinal pathogens).
What is airborne transmission?
Disease is transmitted by droplets or dust (e.g., respiratory pathogens).
What is vector transmission?
Disease is transmitted by living agents such as insects, ticks, mites, fleas, rodents and other animals (e.g., skin and blood-borne pathogens).
What is virulence?
The relative ability of a pathogen to cause disease and can be quantified by experimental studies to determine the infectious and lethal doses.
What is the infectious dose?
The infectious dose (ID) is the number of bacteria required to produce disease.
The infectious dose varies considerably between different organisms.
- Shigella dysenteriae* – the causative agent of dysentery has an ID of 1–10 bacteria.
- Salmonella typhimurium* – a causative agent of food poisoning has an ID of 1,000,000.
What is the lethal dose?
The lethal dose (LD) is the number of bacteria required to kill.
This is usually determined in laboratory animals and is expressed as the number of bacteria required to kill 50% of the population i.e., the LD50.
The route of entry of the pathogen into the body affects the dose.
- S. typhimurium* injected intravenously into mice has an LD50 of 10
- S. typhimurium* given orally to mice has an LD50 of 5,000,000.
What is a nosocomial infection?
Nosocomial infections are those infections that are acquired by patients as a consequence of hospitalization.
This occurs in about 5% of all patients admitted to hospital. In the U.S., there are approximately 2 million nosocomial infections leading to 100,000 deaths each year.
What factors may result in nosocomial infection?
- hospitals treat patients with infectious disease, and these patients may be reservoirs of pathogenic organisms
- patients are exposed to numerous pathogens in a hospital setting, many of which may be resistant to antibiotics
- patients often have weakened immune systems (i.e., they are a compromised host) which make them more susceptible to infection
- pathogens can be transferred from patient to patient by hospital staff or visitors. Often these drug resistant pathogens can be found as the microbiota of the hospital staff.
