From the white patches and how fast the fish are dying I think it's probably Columnaris. NTD is a possibility, but leaning more toward Columnaris. But as mentioned, without more description, symptoms or pictures, we are all just guessing.
There has to be other symptoms, just you might not be aware of (they may just be subtle signs).
Do any of the fish have deformed spines? Any open sores/exposed flesh that appear to be rotting/enlarging? Any ulcers? Any fuzzy/cottony white growths?
If it's Columnaris, that is a aerobic gram-negative bacteria, and Kanamycin (found in Seachem KanaPlex) is regarded as one of the best antibiotics for that. In studies though, it has been shown that the med is most effective at curing Columnaris when combined with another antibiotic, such as Nitrofurazone (found in API Furan 2, most effective at 7.2 pH or lower), Triple Sulfa (better for high pH) or Tetracycline/Oxytetracycine/Minocycline. From studies, Kanamycin combined with Nitrofurazone has produced the most successful results. To slow down Columnaris progression, cooler water temps slows it down. And I believe I have heard adding some salt also slows it down. A UV can help some with general fish health and maybe kill off some free floaters, but don't expect it to cure this outbreak, and remember to turn the UV off when you are using meds.
NTD is said to have no cure. Since you are unsure what the disease is, I would separate all fish that are showing symptoms. The recommended meds above are broad-spectrum meds and can treat a wide variety of diseases, so it has a fair chance of curing whatever the disease may be (doesn't treat everything though). Turn on the UV (stop once you are treating the tank with meds) and remove any dead fish ASAP. If other fish consume the infected fish, they will contract the disease as well. If there are ulcers and they pop, spores can spread and infect other fish.
This is all assuming it's Columnaris or NTD. It could be something else entirely different and the whole tank is infected. It could even be deaths for other reasons non-disease related (stress, poisoning, etc.). We can't really tell without more info.