Are there levels of contamination in a beer or is it a case that it's either "contaminated" or "not contaminated"?

I am trying to understand the nature of contamination in beer. I find it difficult to imagine (considering there are wild yeasts and moulds and micro-organisms in the air - aren't there?) a scenario where a beer is 100% free of contamination, or is this normally the case with a properly brewed batch? Or is it a case that when contamination is lower than some threshold we don't notice it and that beer is considered to not be contaminated?

I am interested because being new to homebrewing and educating my palate, particularly with regard to off flavours I often wonder whether I have some kind of contamination. But I would usually discount this if the beer is question is pretty drinkable, because I would have assumed that contaminated beer would REALLY stand out and not really the kind of thing easily confused with an off flavour, but considering what I was asking about above; could it be possible to have a small amount, or even a tiny amount, of contamination being responsible for off flavour if we define contamination as above a certain ( detectable ) threshold?

If that's the case then is it a case that we're trying to reduce contamination rather than avoid it, or is it possible to avoid it altogether - and is it a case that minute contaminations are common but to the degree that we eradicate these we are likely to get better tasting beers?