= Latent period
{disambiguate=epidemiology}
{title2=$L$}
The <latent period (epidemiology)> is the interval from infection until the infected host becomes infectious. The <incubation period> instead ends at symptom onset. Neither interval is defined by the time of a diagnostic test.<Household Transmission Investigation (HHTI) template protocol for respiratory pathogens with pandemic potential by World Health Organization>[Section 1.3, printed p. 8 and Figure 1: latent, incubation and generation intervals]{ref}
For an illustrative infection beginning on day zero, infectiousness beginning on day two and symptoms beginning on day five give a two-day <latent period (epidemiology)> and five-day <incubation period>. The host can transmit during the intervening three days despite not yet having symptoms. Changing these relative timings changes which observation-based interventions can interrupt transmission.
In an <SEIR model>, the exposed compartment contains infected hosts who are not yet infectious. A transition term $\sigma E$ gives independent exponentially distributed latent durations of mean $1/\sigma$. A fixed latent duration $L$ is a different model: all hosts infected together enter the <infectious compartment> after the same delay. Equal means therefore do not make the two latency assumptions dynamically equivalent.
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