What causes these spots?
 

I have a Miltassia C.M Fitch "Izum" with these spots on the under side of the older leaves. They were like this when I purchased the orchid from a retailer in my area. I assumed it was just a fungus issue and I treated it. The new growth looks nice and clean so I believe I might have fixed the issue. Just for my own knowledge what causes these spots in the attached photo? What would be the best way to treat it if it shows back up again?

Yeah, those are fungal spots. Usually not fatal, but ugly.

Try not to wet the leaves and they should not be a threat.

Also, brighter light can help keep the fungal issues down with UV cooking the fungus and plant tissues growing stronger.

It is best to buy plants with clean leaves to begin with next time.

I had the same plant, but not a single spots on the leaves and stayed that way for a long time I had it.