Question on infections

First off let me start by saying that this isnt an "I ruined my beer thread". So far everything looks ok in my secondary fermenter but I wont know until bottling. This kind of leads me into my question, why is it that you can only tell if your beer is infected after it has been bottled/aged.

I would think that if the beer was going to be infected you should see signs of it in the primary or secondary fermenter before bottling. Is the infection usually caused by dirty beer bottles, improper filling techniques, or some other reason. I am just trying to fgure out why you basically have to wait until the very end to have your heart broken and find out you have an infected batch.

There are a bunch of infections that you can see during the fermentation process, and others around here will even be able to identify what kind of infection it is by just looking at a picture or a simple description of the smell. As far as source of the infection,it could come from anywhere that sanitation was not done properly (old siphon tubing, scratches in brew bucket are common culprits).

I think the question as to why you have to wait is partly because the infection may come from the bottles (in which case be careful of bombs) or could just be misconstrued information. Many infections aren't necessarily a bad thing and some brewers nfect their beer on purpose (I plan on "infecting" my next IPA) to get a different flavor profile. The only thing is these infections can lead to very strong, undesirable tastes initially, but mellow over time.