Necrotic spots rapidly spreading through out this vanda tessellata
 

Necrotic black spots rapidly spreading throughout this vanda tessellata plant. Is this a symptom of a virus or bacterial infection. Any solution to save this plant as its one of my first orchids

This looks like Phyllosticta.
Phyllosticta Leaf Spot

Sure looks like it to me, as well. You can try to treat it, but it looks pretty far along so doesn't show a lot of hope. Make sure it's away from other orchids and watch the others closely.

More information. It looks like you need to throw this away because you won't be able to eliminate the fungus from this plant.
Phyllosticta capitalensis

Thanks!

Did it get cold? If that's the case, you could watch and see if new leaves are OK in a warmer place (and perhaps the black spots will dry up), but do isolate it from other plants.

Yes it did get quite cold since I moved to the mountainous area(1300masl) of Sri Lanka
Temperature dropped from 28-30 to 23-18-15 Celsius.