The <SIR model> is
$$
S\xrightarrow{\lambda(t)}I\xrightarrow{\gamma}R.
$$
The expected infectious duration is $d=1/\gamma$.

Solution

Past exam of the mathematics course of the University of Cambridge / 2025 / iii / Paper 207 / 2 / d / i / Solution

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The SIR model divides a closed population into susceptible, infectious, and recovered compartments. Infection transfers people from $S$ to $I$, and recovery transfers them from $I$ to $R$.


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