You have to treat your entire tank if that is the case. From the sounds of it you have multiple fish living in the same tank? Yes, ich can kill fish if left untreated. If you're just now noticing it, you probably have a few days, though it is hard to tell. There are medicines to treat ich at any local pet store in the aquatic section - pick one and follow the directions. I would probably do a water change, using a stress coat water conditioner to boost the fish's immune system. Also, if you have a heater, SLOWLY raise the temperature of the tank a couple of degrees. Ich thrives in colder water.
Also, these articles may help:
Also, google images of ich to make sure you're not giving your fish medicine unnecessarily. Hope that helps :)