The egg is typically deposited by the egg carrier on or in the host and upon hatching the larva begins feeding on the host gradually killing it. In the case of the fictional Xenomorphs, the 'chestburster' larva is not known to actually feed on the host during this incubational period (though it is assumed it does, as the affected become hungry and tired) but does eventually subject it to a dramatic death. Once it's ready to move on to adulthood, the endoparasitic xenomorph will violently burst through its host's chest cavity.