6. Epidemiology and Public Health Flashcards
What is an infectious disease?
- An illness due to a specific infectious agent or its toxic product that arises through transmission of that agent or its products from an infected person, animal or reservoir to a susceptible host, either directly or indirectly through an intermediate plant or animal host, vector or the inanimate enviornment
Define:
- An agent;
- Infectivity:
- Pathogenicity:
- Virulence:
- An agent: the entity necessary to cause disease in a susceptible host e.g. bacteria
- Infectivity: the capacity to cause infection in a susceptible host
- Pathogenicity: the capacity to cause disease or clinical symptoms in the host
- Virulence: the severity of the disease that the agent causes in the host
Describe:
- Source of an agent:
- Portal of exit:
- Portal of entry:
- Source of an agent: where the agent originates, lives, grows and multiplies
- Portal of exit: pathway by which the agent can leave the source
- Portal of entry: pathway into the host (often the same as portal of exit)
What is the difference between direct and indirect transmission?
Direct transmission:
- direct contact with soil, plants or infected people
- Mucous membrane to mucous membrane
- Skin to skin
- Across placenta
- Faecal-oral route
- Sneezes and coughs (note these are not indirect as the source must be in very close proximity to the source)
Indirect transmission:
- airborne proper
- vector borne e.g. mosquito
- vehicle borne e.g. objects
- waterborne
What is the incubation period?
- Period between exposure to the agent and the onset of disease
What is the clinical disease?
- Symptoms and signs of infection
- Notification date in surveillance data
What is the latent period?
- Time from infection until infectious period begins
What is the infectious period?
- Time during infection that a person can transmit disease
What is:
- The index case
- The primary case
- Secondary case
The index case:
- The first case in the population (may not always be identified/notified)
The primary case:
- The first identified and/or notified case
- May refer to all cases originating from a point source exposure
Secondary case:
- Case resulting from subsequent transmission
What is reproduction rate?
- The number of cases one case generates on average over the course of its infectious period
- The larger the Ro the harder the disease is to control
- More difficult to detect in a heterogenous population (a population in which not everyone is susceptible)
What is:
- Immunity:
- Herd immunity:
Immunity:
- The capacity of a person when exposed to an infectious agent to remain free of infection or clinical illness
Herd immunity:
- The immunity of a group or community
- Based upon resistance to infection of a high proportion of individual members of the group
Describe:
- Sporadic disease occurrence:
- Endemic disease occurrence:
- Hyperendemic disease occurrence:
- Holoendemic disease occurrence:
- Sporadic disease occurrence:
- a disease that occurs irregularly/infrequently in the population - Endemic disease occurrence:
- constant presence of a disease/infectious agent within a given geographic area/population group - Hyperendemic disease occurrence:
- the constant presence of a disease at a high incidence/prevalence and affects all age groups equally - Holoendemic disease occurrence:
- high prevalent levels of infection early in life, affects most of the childhood population, adults show less evidence of the disease
What is:
- An epidemic/outbreak?
- A pandemic?
An epidemic/outbreak:
- Occurrence of cases in a community or region in clear excess of normal expectancy
Pandemic:
- A very widespread, often global disease outbreak
Describe the disease Salmonella in terms of disease outbreak:
Causative organism: Salmonella enterica
Diseases:
Local- gastroenteritis
Systemic- typhoid (Salmonella enterica typhi)
Reservoir:
- GI tract of many species of animal
- Environment contaminated by feces
Source:
- Contamined food or objects or from infected animal/person
Portal of exit:
- GI tract
- Fomite
Portal of entry:
- Oral: ingestion of Salmonella bacteria
Transmission:
- direct
- indirect
Characteristics of disease timeline:
- Incubation period: 12-72 hours
- Clinical disease: 4-7 days
- Short latent period, very long infectious period (people can be carriers)
Immunity:
- not always induced due to numerous serotypes
Describe Ebola in terms of an outbreak disease?
Causative organism: ebola virus
Resevoir: bats?
Source: bodily fluids of sick/dead person, contaminated objects, infected animals
Transmission:
- Direct and indirect
Portal of exit:
- Bodily fluids secreted by skin and mucous membranes
Portal of entry: penetration of skin and mucous membranes
Timeline of disease:
- Incubation period- 2-21 days (lines up with latent period)
- Clinical disease: death-20 days
- Infectious period: from when clinical disease begins to 2 days post
Immunity:
- no carriers
What are some features about the Zika virus and its transmission?
- Caused by a flavivirus
- Reservoir: monkeys and rodents
- Source: blood and bodily fluids
- Transmission: direct (sex, perinatal transmission), indirect (mosquito vector)
- Disease: mild but causes microencephaly in developing fetuses
- Immunity: carriers exist
What is a cause of a disease?
- A cause is a factor/agent that plays an essential role in producing an occurence of the diesase outcome
What is a:
- necessary cause
- sufficient cause
- component cause
- Necessary cause:
Outcome only occurs if factor has operated - Sufficient cause:
factor that inevitably results in the outcome - Component cause:
factor that contributes to outcome but not sufficient to cause disease on its own or necessary to develop the disease e.g. smoking is a component cause in lung cancer
Describe the causal relationship of plasmodium and malaria:
- Not sufficent: some people with plasmodium never develop malaria
- It is neccessary: malaria does not develop in the absence of plasmodium
- Other components: host immunity and genetics
What is causation?
- A factor that is a cause of an event- it alters the frequency of the event occurring
Describe the casual relationship of HPV and cervical cancer:
- Not sufficient: some people with HPV never develop cervical cancer
- It is neccessary: cervical cancer does not develop in absence of HPV
- Other component factors: immunity
What is causal inference?
- Determining that exposure is linked to developing disease
What are the Bradford-Hill Causal guidelines:
- Temporality:
- cause must precede disease
- only essential criteria - Strength:
- strong associations more likely to be causal - Consistency:
- association exists in many different populations - Dose-response relationship:
- increased dose of exposure leads to increased risk of disease - Biological plausibility:
- The relationship is biologically credible - Specificity:
- one cause = one effect
- not used today
What are experimental vs observational studies:
- Experimental:
- Manipulation of the exposure through intervention in controlled environment
e. g. RCT - Observational:
- No intervation
e. g. cohort
e. g. case-control
e. g. cross-sectional
e. g. ecological
