Where did you get Annabelle? Ziv was recently diagnosed with chlamydia, too. We got him from a shelter in January.
I did a bit of digging around on the 'net, and apparently, chlamydia is one of the major causes of Upper Respiratory Infections in cats. So, it's passed just like a URI--cats together in a shelter, cattery, or household can get it through normal interactions. So far, neither Eva nor Chesty has shown any signs, but the specialist said it's probable that they will get it, eventually, so I'm keeping an eye on them.
The specialist said that it's possible to get rid of chlamydia, if you treat it systemically. In other words, you can't just treat the eye flare-up, you have to give strong internal antibiotics. Ziv is currently in the middle of his three-week course of Zithromycin. He's also taking Lysine just in case it turns out he has herpes instead of/in addition to the chlamydia. The antibiotics are pricey (around $120? from CVS--you have to get them at a human pharmacy)--but Ziv was pretty miserable before, and he's been SO much better lately. His eye is tearing much less, and it doesn't look pink anymore. He also just seems generally happier, more cuddly, etc.
So, if you got Annabelle from a shelter, or if she regularly interacts with other cats--outdoors or something--that's how she got it. If I were you, though, I would try to get Zithromycin for her. I'd imagine that, unchecked, chlamydia could really wreak havoc on her immune system, and also could infect your other cats. We had to take Ziv to a specialist (an opthalmologist) to get the diagnosis (our regular vet had no idea what was wrong), and he's the one who prescribed the treatment. Good luck!