There are occasional such smells on the B757s I fly. I don't know whether the odour is life threatening or not but the smell described (sweaty socks) is positively awful. If the same smell were evident through the ventilation system of my car I'd definitely whisk it into the garage for fault finding analysis! As Neil Armstrong says above, our company tell us the smells are normal but they're not... I've been flying the 757 for 14 years and as far as I can tell the problem is only noticeable on some aircraft!
I hope the publicity of this topic may encourage the CAA, the Government, and the COT to come clean once and for all and order airline operators to fit smell monitoring equipment on board so that pilots, cabin crew and passengers can know for certain if there are health issues at stake.