DC FEMS quickly loaded the person into an ambulance, which headed West on Florida with an MPD escort, presumably going to George Washington Hospital.
WTOP has the first news story online, with little detail beyond what was evident at the scene. MPD and Transit Police are investigating, and the Metrobus driver will undergo drug and alcohol testing.
From what I saw at the scene, it looked unlikely that the pedestrian was in the crosswalk (see diagram). The bus was all the way through the crosswalk, and people were still able to use the crosswalk. The point of impact appeared to be roughly a bus-length east of the intersection. Pure speculation, but I'd guess the person was crossing the street on a path that goes directly to the Rite Aid, rather than crossing the street at the crosswalk with the island. I've been guilty of jaywalking in that exact same spot.
There's no information yet on the extent of the injuries other than they are "life-threatening." It was a bit terrifying to see. My thoughts go out to the victim and the family. Scary stuff.