Zygo. mackeyi Yellow/Brown Leaves

[risagrey]

I recently purchased a Zygo. mackeyi. Just the other day I noticed that two of the leaves were turning yellow and brown. I cut them down and then sprayed it with fungi spray. The next day the whole 2 leaves were yellow and brown. I cut them all the way down this time. It has a bud and three other leaves and I am afraid it will spread. Does anybody know what this would be? Could it be a virus? I have it away from my other orchids.

Thank you for your help.
Tanya