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...
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Interesting thanks
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Here's video footage of it in action. All the clone in this clip are infected. All of them.
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Creepy, so what is the purpose of this "creature"? Is it sentient?
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Personally I don't know, I think of them as like little bits of the Geonosian Queen, as she uses them to force people into submission. She was the entire reason Geonosis rose up for a second time out of fear of the worm. But it might still be sentient in the fact that it can mimic people's emotions and personality perfectly.