Allthough it's important not to rule out Marek's until you know for sure, this does sound a bit more like botulism than Marek's. I am not a professional, so take this opinion with a grain of salt, so to speak.
Botulism toxins can kill birds quickly, like what you experienced, or not-- it depends on how much toxin the bird ingested. If it is indeed botulism, your sickly hen that is still alive may not have eaten as much toxin as the ones that passed away, and it is likely she will recover. Botulism can occur, as mentioned, in rotting vegetation, but is more often found in rotting carcasses and resulting maggots and larvae that feed on the rotting flesh can eat and store the toxin in their bodies with no ill effect, but if a chicken or duck were to eat a gut-load of these maggots, the bacteria can cause botulism in them.
Botulism needs anaerobic conditions-- that is, the lack of oxygen, and so it is more commonly seen in situations such as rotting vegetation or animals at the edges of waterways and in swampy conditions(this is how ducks so often get it) and sometimes in places such as compost piles if they are not turned.
Here's some reading on
Botulism:
http://www.merckmanuals.com/vet/poultry/botulism/overview_of_botulism_in_poultry.html
http://www.avianweb.com/botulism.html
If you suspect
Marek's, here is some information to get you started:
https://www.backyardchickens.com/a/the-great-big-giant-mareks-disease-faq
Either way, keeping them on healthy feed, keeping their stress low, and keeping them away from any potential sources of botulism bacteria, for now, is a good idea. Again, if it's botulism, your surviving birds (even the sick one) are unlikely to die directly from the toxins at this point as it has been over 24 hours, so flushing is probably not necessary.
Good luck to you, I am hoping for the best!!
