Lecture 2 - Microparasites + SIR Models

What are the three compartments in the basic SIR model?

Susceptible (S), Infectious (I), and Recovered (R).

What does S represent in the SIR model?

The number of susceptible hosts that can become infected.

What does I represent?

The number of currently infectious hosts capable of transmitting the pathogen.

What does R represent?

The number of recovered hosts who are no longer infectious and are assumed to have immunity in the basic SIR model.

What does N = S + I + R mean?

The total host population is divided among susceptible, infectious, and recovered individuals.

What happens to S when a susceptible host becomes infected?

S decreases by one and I increases by one.

What happens to I when an infected host recovers?

I decreases and R increases.

What does β represent in an SIR model?

The transmission coefficient, representing how effectively infection is transmitted through contacts.

What does γ represent?

The rate at which infected individuals leave the infectious class, primarily through recovery in the basic model.

What does 1/γ represent?

The average duration of infectiousness.

If γ increases, what happens to infectious duration?

Infectious duration decreases because duration is approximately 1/γ.

If β increases, what happens to transmission?

Transmission increases, causing infections to spread more rapidly.

What is an epidemic curve?

A graph showing the number of infected individuals over time.

What causes the number of infected individuals to increase during an epidemic?

New infections occur faster than infected individuals leave the infectious class.

What causes the number of infected individuals to decrease?

Infected individuals are being removed through recovery, death, or other loss processes faster than new infections occur.

What does dI/dt represent?

The rate of change in the number of infectious individuals over time.

What does βSI represent?

The rate at which susceptible and infectious hosts generate new infections under the density-dependent formulation.

What does γI represent?

The rate at which infected individuals recover or otherwise leave the infectious class.

How would increasing β change an epidemic curve?

It would generally produce faster transmission and a more rapid increase in infections.

How would increasing γ change an epidemic curve?

Infected hosts would leave the infectious class faster, shortening infectious periods and reducing transmission.

Why is S important even when β and γ stay constant?

As susceptible hosts are depleted, fewer hosts remain available to become infected, reducing transmission.

Why does an epidemic eventually decline even if the pathogen has not disappeared?

The susceptible population becomes depleted, reducing the number of new infections.

What is the difference between latent and infectious periods?

The latent period is the time after infection before an individual becomes infectious. The infectious period is the time during which the individual can transmit the pathogen.

Why can transmission occur before symptoms appear?

The infectious period can begin before the symptomatic period, allowing individuals to transmit before realizing they are sick.

Why can natural selection favor transmission before severe disease occurs?

A pathogen that kills or disables its host before transmission has fewer opportunities to spread.

If an infected individual recovers very quickly, what happens to transmission potential?

It decreases because the individual has less time to transmit.

If the susceptible population is reduced by vaccination, what happens to transmission?

Transmission decreases because there are fewer susceptible hosts available for infection.