People act like HITH is the end of the world it's not. It can and will heal if you know how to care for fish. Even deep holes will heal up but the deeper it is the longer it will take. And yes if it is bad enough you fish might Never look the same again but will still look good. I have rescued a few fish now with hith. One had a very light case of it it cleared up in a few weeks with good food and extra wc. Then my uaru I just rescued had a pretty bad case it's been a month and they are 90% healed. I will get some new photos to show before and after. I need to update that thread.
But if it's a few small holes keep the water perfect daily water changes and if you can get vitamines they work wonders.
No it will not spread. Most hith is caused by poor diet, stress, poor water and even carbon dust has been linked to it. Yes a parasite has been thought to cause it but that is not the common cause.