Nasty! Nasty!
Ideally you should get a sample of diseased tissue to a path lab to find out just what is causing the problem, otherwise you're fighting in the dark but I guess that probably isn't practical. Also it would take a lab some time to culture the beastie and from what I've seen time is short.
IMO you need to determine firstly whether the problem is environmental, chemical or a pathogen.
Environmental is unlikely from what I've read so far if you have controlled conditions. Yes, incorrect environmental conditions can cause problems something like that but unless there has been some dramatic change recently then it's not likely to have afected so many plants so badly. Chemical damage can be ruled out if you don't use chemicls and your water is OK and not too hot or cold.
Some pathogen seems most likely. However, the fact that it's spreading so fast is interesting. Are your plants standing in water? (Sorry if you've already told us that but I'm writing this off-line to upload later).
If it's a fungual infection I would say probably not a root rot. Phythium and Phytopthera spp. are more common than you might think and produce a variety of symptoms in Nepenthes. However, the usual symptom is a yelowing of the leaves and brownish discoloration of their undersides. Not what you are seeing here.
So, IMO some sort of crown rot perhaps, Most probably bacterial. But how is it spreading so fast? If you are watering from above is there a chance of water splashing from one plant to another? That would do it!
As to treatment, I know what I would do here but my conditions/situation is entirely different to yours so I won't even suggest it. The best advice has already been given to you - trim off all affected parts BUT (if it's not already too late!) make sure you disinfect your scissors or whatever you are using after each cut otherwise you'll definitley risk spreading the infection around. Also don't handle infected plants and then even touch a clean plant. Be bold! Cut a little below the infected area to make sure you get it all. Think of it as a gangrene, if you leave any behind it'll keep coming back.
Not knowing whether it's bacterial or fungal isn't necessarily a problem if you use some generic disinfecting agent that isn't too phytotoxic - such as Physan 20 if you can get it. You can treat the whole plant after trimming (if you dare) or just treat the area you have just trimmed.
BTW, I'm afraid you N. jacquelineae pitcher won't develop properly without an intact lid. They need to inflate with air to reach their proper size and shape.
Good luck!