So do the inactive carrier bodies not need sustenance? They said they'd be back but if something stops them they could risk his sister dying. Also, I wouldn't really trust the firefighter guy. While the objective of the group is to provide people safety, it is also to round people up to avoid the spread of the infection. Thus, the firefighter guy may not hold true to this promise, since a part of his job is to get people together.
Given that this manga isn't really logic-based, though, this could all just be ignored in service of moving the story along.