Latent period (epidemiology, $L$)

The latent period 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.^[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 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.