: Mostly, I'm thinking of Infection Forms and Carriers. They were more
: aggressive in the original game, and there was a metric crapload of them.
: In Halo 3, infection forms spent most of their time wandering around on
: the walls than attacking.

I think in each game their swarming behavior was different. In the first two games they tend to stay where they were until a player got close enough, then follow the player (along with any infection forms were close enough to the ones that noticed the player). In Halo 3, it seems like there's some critical mass that varies between levels and parts of levels, and if there are more than X infection forms, all of them within a certain distance come for the player(s), until there aren't enough any more, with certain considerations made for distance with smaller groups and individual infection forms. Also they jumped more before Halo 3.