Boost is an ability that causes the specified unit to increase X attack for just one turn, at the start of attack in its battle cycle. Please notice the abilities Orc ally and Rage is more similar to Spearhead than Boost, although the attack increase is also in the same turn.
The Ability Sap will decrease the boosted attack first, while disease will decrease the original attack. This makes boost giving protection only to Sap ability. However,
Many abilities feature Boost as part of the function.
Orc Ally and RageEdit
The boost effect will last forever, given the required conditions are met. However, the boosted attack would act like no time to decrease, as if the condition is met, the boosted attack from last turn is removed just before the new turn boost is added.
This version of Boost will have its attack removed after the unit's attack in battle cycle, thus, not protecting from any Sap effect.