question about chlamydia

[so-much-udder.livejournal.com]

so I've been doing some reading on chlamydia, and something that keeps coming up is how most people don't even know that they have it, and so I was wondering why that was. I'm assuming it would show up on a standard STI screening, but does it not manifest itself for a while? I'm just wondering because I know it can cause PID and so obviously I wouldn't want to leave it dormant out of ignorance; so how long it would take to show up on an STI test? and I guess do people just not know because they don't get tested? or is there no way to know unless it gets serious?