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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.

Suppose that newly infected members of a biological population pass through a noninfectious stage of fixed duration $\tau$ before becoming infectious. This delay is a latent period, whereas the incubation period ends at symptom onset. The two endpoints need not coincide.^[ref]

Let $b(t)=rI(t)S(t)$ be the rate of new infections in the specified compartmental model. Assume no death or other removal during the noninfectious stage. At an intervention time $T$, all infections acquired during the preceding $\tau$ time units are still pending entry into the infectious class, soFor a smooth recent incidence history, a Taylor expansion about $T$ givesThus the leading pipeline correction is proportional to the recent infection rate, rather than simply to the number currently infectious. A rapidly varying incidence history requires the integral instead of this short-delay expansion.

An ideal intervention stopping all new transmission at $T$ does not reverse these already acquired infections. Under the assumptions above, exactly $E(T)$ additional entries into the infectious class occur after $T$, over the next $\tau$ time units. They must be added when the reported cumulative count at $T$ records infectious-class entries. If that count already records infections at acquisition, the pending individuals are already included and must not be counted twice.

The definition of the susceptible individual compartment also matters: a genuinely uninfected susceptible class loses an individual at infection, while a model grouping not-yet-infectious individuals with susceptible ones postpones that recorded class transfer. The same biological delay therefore appears at different places in the bookkeeping. The assumption of perfect transmission removal is a model idealization rather than a clinical intervention rule.