If I recall correctly, Carlito in DR1 somehow infected some kids and let them get adopted all over America. Now, I don't recall that being ever mentioned in this game, but then again, I think they might be using Zombrex now so a nation-wide zombie outbreak was avoided.
Does the game ever adress this? I played DR1, DR2 and DR2:OTR and I don't think it's ever addressed, but I might be wrong.
An epilogue states that Frank West later managed to escape the town of Willamette with information about the situation. He released the news, causing a worldwide fervor that leads to the United States government admitting partial responsibility for the outbreak in Santa Cabeza. However, they claim that the Willamette outbreak is the work of a terrorist group (which is technically true), and they are not to be held responsible.
The people of the world soon allowed the Willamette incident to fade from their minds. Although Carlito's plan to use the orphans to infect the entire United States has not been confirmed or proven false, it remains a chilling possibility.