Okay for anyone interested, here is what went down. I am guessing pretty standard for anyone who knows about any care but I will still share if anyone has the same issues and this turns out helpful.
I took the orchid out of the pot, cut the spike, all the rotten and completely dried out roots and then put it for a bit of a swim in my bathtub. I also added a bit of a dishwashing liquid as kindly suggested.
Afterwards, I took disinfected shears and cut off the diseased leaves. The plant had a keiki at the end of one of its spikes. The keiki also had this disease which I had no luck in discovering what it is but I did the same with the keiki. I gently wiped the leaves and where I cut the leaves, I also put cinnamon.
Then I repotted both and put them in a nice just bright enough spot.
This is what happened since then:
The keiki is growing new roots and new leaves, the same happened with the mother plant. The new leaves that are growing do NOT have the disease. I cannot even tell you how happy I am. And the mother plant has now started growing a new spike to bloom for the first time in years. It happened so fast, I am beyond delighted.
Ever since this has happened, I have had friends and my grandmother bring me their orchids, asking for help. I wish I could be an orchid doctor for a living.