Limerance is closer to obsession than infatuation. I am a limerance sufferer (I was the OW). If you've not experienced it, it is difficult to explain. Think of trying to explain a deja vu (if you've had one), to someone who has never had that experience. It is almost impossible.
Limerance has benefits - it gives you a sky-high high better than any kind of drug you could imagine - just from being in the presence of a person. It makes you feel utterly alive. But it is equally devastating - characterised by seriously, life destroying intrusive thinking.
A typical feature of limerance though is that the love object is slightly out of reach... limited time spent together. It rarely is sustainable long term in a day-to-day relationship warts and all because it is fundamentally founded on clouds.
You may find limerance in an affair but my guess is that mostly it would be in the OW/OM rather than in the M partner.
However, to answer you question about affair fog - I've said this before but an affair is very exciting at the start because it is a fantasy relationship; it is unreal.
Both partners have lots of sexual chemistry - which is normally exciting even in a "both single - could this go anywhere" set up.
Add into that for the M partner feeling that someone (usually) younger and desirable wants them plus the newness of a new partner after years of fidelity.
Add into that for an OW/M (who may not know the M person is married) a pursuit like they have never experienced. The M partner chases like hell, with hindsight, because they have nothing to lose - they have a husband/wife so rejection doesn't really matter. This is so flattering and makes you feel like this is your soulmate; in fact it is just the security of their primary relationship. No one single chases a person like that - but until you've been a victim of this, you don't realise it.
Add into all of that for the MM/W, it is exciting because it is forbidden and they know they shouldn't be doing it.
And then the icing on the cake is that it's all romantic dinners, expensive hotels, champagne and roses - NOT headaches, farts, dirty laundry, snotty noses etc.
It's not real - so yeah there is a fog. A fantasy fog.