i believe if it was velvet and not treated immediately, the fish would stand little chance of recovering. fresh dips and copper i think are two methods.
i agree if you have a hippo that shows no signs over time,,chances are great that your system is free from c.irritans.
in fact, if it was velvet and no other fish have symptoms, then you're probably not dealing with that either.
without a pic,,tough to help diagnose.
my tank is going through a lengthy fallow period right now to make sure.