The developers ensured that this change, as well as the knowledge of Ellie's immunity, was kept secret prior to the game's release to surprise players.
Losing everything that you know.
Suddenly, Tommy pulls into their driveway and they jump into his car where they begin driving out of town to get to safety, but some routes are blocked and others clogged with traffic.
With no other options, they fight their way out the back of the school, having to kill the strongest of the Infected - a - to do so.
As it turns out, there are various notes scattered amid the surrounding buildings alluding to the fact these poor souls had been herded here to live out their days in captivity, but the partially-written tales of misery and woe contained in said notes remain unfinished until you arrive to emphatically finish them--to put the nail in the coffin, so to speak.
In order to really forgive someone, that person has to show that they can be forgiven.