Fungus are rare--bacterial infections are not. Most likely, the fish was either injured or kept in poor water conditions, or had an ailment which reduced it's slime coat, allowing a bacterial infection to set in. Depending on the condition of the fish now, it may recover (ie, in good water, still eating and behaving normally) without treatment, or it may not. If poor water conditions still exist, improving them will help the fish out. If other fish are harrassing this one, it will be less likely to improve. Removing the fish to a quarantine tank, and being able to medicate if it worsens or fails to improve, would be my best advice.