Infections of Populations: History and Epidemiology Flashcards
What are some reasons for seasonal variability n certain viral infection prevalence?
virus stability at different temperatures/ humidity levels
Change in human behaviour - e.g indoor during winter
Vector life cycle - e.g mosquitoes during rainy season
What are Koch’s postulates?
microorganism is associated regularly with disease
microorganism isolated from diseased host
if microorganism inoculated into a new host, it should cause disease
microorganism can then be isolated from this second host
Not necessarily as applicable to viruses.
- very often viral infections may not cause disease
- Viruses such as HPV/ HBV cannot be cultured in cell lines
What is the definition of incidence
Number of new cases, within a population, in a specified time
e.g Influenza 10 cases/ 1000 people/ year
Usually most useful for describing acute infections
What is definition of prevalence
Number of current cases, at one moment in time
Usually most useful for describing chronic infections, in which disease onset is not easily determined
High incidence + low prevalence - e.g Influenza - people who are infected recover quickly or die. So we have lots of new cases, but not that many cases long-term
Low incidence + high prevalence - e.g HBV - chronic infection, not many people infected in short time, but overall a lot of cases
What is definition of prevalence
Number of current cases, at one moment in time
Town with 100 000 residents
Virus causes infection of 10 000 people
7000 people develop illness
500 people die of infection
What is the incidence?
10%
The incidence of this infection is the number of people infected divided by
the population (10,000/100,000, or 10%).
Town with 100 000 residents
Virus causes infection of 10 000 people
7000 people develop illness
500 people die of infection
What is the morbidity?
Morbidity is the number of individuals who became ill divided by the number of infected individuals (7,000/10,000, or 70%).
Town with 100 000 residents
Virus causes infection of 10 000 people
7000 people develop illness
500 people die of infection
What is the mortality rate?
Mortality is the number of deaths divided by the number infected (500/10,000,
or 5%).
Basically - if you get infected, how likely are you to die?
This is different to case-fatality ratio
Town with 100 000 residents
Virus causes infection of 10 000 people
7000 people develop illness
500 people die of infection
What is the case-fatality ratio?
The case-fatality ratio is the number of deaths divided by the number of individuals with illness (500/7,000, or 7.1%).
What are explanations for seasonal variation in some infections?
Population density - crowding during winter or on cruise ships
Vector life cycle - e.g summer allows mosquitoes to breed in Europe, where they die off during winter
Climate - change allows vectors/ hosts to migrate. Dry/cold can help influenza virus survive longer on surgaces
Host changes - possible that changes in host circadian rhythm, or mucosal surface thickness changes in summer/ winter may affect host susceptibility. Cold winter could dry human skin, and allow mucosa to be breached by virus
What are physiological differences that can influence risk of infection?
Male more likely to have infections - unclear why. Possible hormonal changes
Infants less likely to have severe disease - e.g rubella, VZV, HBV - likely poor immune response
Malnutrition - unable to fight off infection. E.g protein/ vit A deficiency increasing measles mortality
Individual changes- e.g CCR5 deficiency in HIV
