I've done a bit of research on the Neon Tetra Disease (Plistophora hyphessobryconis) and it is contagious to other Characins!!
Its a parasite that causes muscular degeneration even disintegration. It can infect other characins when the kidneys are infected and urine gets into the tank. Also when the 'pansporoblasts' erupt on the skin it can infect the tank. Spores taken up with the food release the amoeboid stage in the gut, whence they penetrate the gut wall and subsequently invade the muscles.
Diagnosis: The destroyed muscles shine whitish through the skin. The signs are most distince in the Neon Tetra, in which both the shiny green/blue and the red stripes may be broken up by it. The milty-white places, which lie under the skin, also occur in other fish...
Both Cardinals and Neons can carry it... I would avoid both but especially Neons...