## SIR model with sinusoidal forcing (page 160)

A large number of species experience seasonal forcing, either due to host aggregating a certain times of the year, or due to transmission rates being influenced by climatic conditions.  We begin to analyse such seasonally forced systems, by making the transmission rate β vary sinusoidally about its mean value. This system of equations was first studied by Klaus Dietz in 1976, when he showed the complex dynamics that could arise from including a small amount of sinusoidal forcing. The basic equations are as follows:

It should be noted that large values of β1 can lead to large-amplitude fluctuations and numerical errors.
Parameters
 β0 is the mean transmission rate β1 is the amplitude of sinuoidal forcing ω is the frequency of the oscillations. We set ω=2π/365, such that oscillations are annual. μ is the per capita death rate, and the population level birth rate. γ is called the removal or recovery rate, though often we are more interested in its reciprocal (1/γ) which determines the average infectious period. S(0) is the initial proportion of the population that are susceptible. I(0) is the initial proportion of the population that are infectious.
All rates are specified in days.

The programs can return either standard time-series, or bifurcation plots. Bifurcation plots are achieved by setting β1 to be a vector in the Matlab code, or by setting Num_Bif_Steps in the parameter file for the C and Fortran code.

Requirements.
All parameters must be positive, β1 ≤ 1, and S(0)+I(0) ≤ 1

Files
C++ ProgramPython ProgramFortran ProgramParametersMATLAB Code.

 Questions and comments to: M.J.Keeling@warwick.ac.uk or rohani@uga.edu Princeton University Press Our research web pages: Matt Keeling      Pejman Rohani