In our flock we have never had more than one bird with Marek's (or whatever neuro problem it is, and there are a few conditions that have similar symptoms) get it at a time. Many develop immunity naturally. If your chick hasn't developed additional symptoms by now, it is probably an injury.
in addition to above, please keep a close eye on her. Sometimes Marek's disease and other neurological issues start out as just lameness with no apparent cause. Hopefully it is just a sprain or bruise! Bumblefoot can also present as lameness so as has been said, look for anything unusual on her...