It is best to remove the affected fish and quarantine them too keep an eye on them. Removing them also ensures that should one of them die, the others won't eat the dead fish's flesh, which is how NTD spreads. NTD can also affect other species of fish, mostly other species of tetras though, if I remember it right.
Lots of water changes may help the sick fish if it is not NTD, and only medicate once you have diagnosed them either for fungus or bacterial infection.