Infections of Populations

What is the epidemiology triad model? How do these 3 things affect virus disease outcome/ ability of a virus to cause disease?

• Host

  • Genetic and nongenetic factors

• Virus

  • Ability of a virus to infect cells and cause disease

• Environmental factors

  • Ability of a virus to persist in the environment

Incidence vs Prevalence

• Incidence: The # of new cases of a disease within a specified time period

• Prevalence: The total number of cases of a disease during a specified period

Morbidity

Frequency of illness in a defined population during a specific interval of time

Mortality rate

Frequency of the occurrence of death in a susceptible population during a specified period of time

*mortality rate is a measure of the occurrence of death in general, not just due to a particular disease

Case fatality rate

The proportion of people with a disease that die from the disease ( # deaths / total # with the disease)

Endemic

A disease that is consistently present in a particular area

Epidemic

An unexpected increase in disease cases in a specific area over endemic levels

Pandemic

Disease growth is exponential and involves multiple countries or continents

Sporadic

A disease that occurs infrequently are irregularly

Reservoir

Habitat/host in which an infectious agent replicates/ is maintained

Vector

Animate intermediate that transmits an infectious agent from the reservoir to a susceptible host

Ex. mosquito with dengue

Vehicle

Contaminated food, water, or biological products

Fomite

Contaminated inanimate objects

ex. medical devices, doorknobs

Source

The reservoir, vector, vehicle, or fomite from which a host is exposed to and infected with an infectious agent

Zoonosis

Spread of a disease from an animal host to a human host

Anthroponosis

Spread of disease from humans to animals

Biological vs Mechanical vectors

• Biological: Infected by the infectious agent and supports its replication, transmits through biting and vertically through eggs and larvae

• Mechanical: Virus does not infect and replicate in the vector, external transfer occurs

Direct transmission

Transmission from a reservoir directly to a susceptible host, such as through personal contact, biological droplets, or vertical transmission to offspring

Indirect transmission

Transmission from reservoir via an intermediate source, such as airbone sources like dust, vehicles, inanimate objects, or vectors

Is disease caused by a vector a case of indirect or direct transmission?

Indirect

Vertical transmission + examples

Direct transmission from mother to child, Zika virus

R0 value

Average number of secondary cases resulting from a single primary case in a large population of susceptible hosts

R0 < 1

Disease will die out quickly

R0 = 1

Disease will be maintained in population at endemic levels

R0>1

Greater chance of disease spreading through susceptible populations causing an epidemic

What affects the R0 of a virus?

• Duration of infectivity

• Persistence in the environment

• Ease of transmission

• Virulence

• Number of susceptible hosts

Environmental parameters of viral transmission

• Geography - can be restricted to where a vector or reservoir host lives

• Population density - maintenance of viruses with more limited host range and more virulence

• Climate - Temperature and humidity