They controlled everything the host did but retained it's personality to the fullest. The only time anybody suspected anything after the ship that was infected left geonosis was when the clones started attacking the Jedi hell, the worm even infected a Jedi for a brief amount of time. Also there's no way of knowing how many people are infected, as stated above it doesn't change it's host's personality or memories. It even speaks the same way. So yeah...