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PROFESSOR: In Chapter 3, we will
study epidemiological models,

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which describe the
spread of an infection

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and then the recovery from
that infection in a population.

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And the traditional
epidemiological models

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that we will study
for populations,

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and also adapt to
indoor spaces, involve

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keeping track of compartments
or subgroups of populations,

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such as the number of
susceptible people,

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the number of infected,
exposed, and recovered.

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So for example,
an infected person

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can expose a
susceptible, and then

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the exposed person can
themselves become infected,

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or they may eventually recover.

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And there are various rates
for these different processes.

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And this leads to a set
of nonlinear differential

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equations that describe the
evolution and growth, and then

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ultimately decay,
of an epidemic.