Birds stopped coming very abruptly to feeder with black sunflower seed. How can i be sure there is no virus or pox?

Birds can be forgiving if a feeder is empty for a few days, but a feeder that is consistently empty won’t attract birds. Wild birds won’t starve if feeders are empty since they get most of their food from natural sources, but they also won’t return to an unreliable food source.

https://www.thespruce.com/top-bird-feeding-mistakes-386614

Maybe there is a predator nearby like a hawk or cat.They might have found another food source.Of course, migratory birds leave for awhile.
A friend who is a photographer once set up a blind near his feeder to get images of the birds that visited. His first day out he got nothing.
He was out there for hours and when he finally got fed up, he walked out to see his new cat sitting underneath the feeder. :)

The most cost effective way, would to be simply getting a new feeder…

And yes, a hawk, or new population of domestic cats, could also explain the situation.

Birds cannot detect disease. So. That’s unlikely the problem. But. Not impossible…

I have a Red Hawk family, in my back yard. They seem to come and go, but they will kill anything that flies. I saw a pretty red butterfly one day, and a hawk snatched it out of the air, right in front of me. I still have lots of birds around, but they disappear when a hawk is around…

Replace the feeder, and get new seeds, and see what happens…

A bird feeder provides an ideal target source for birds of prey. Perhaps there is a hawk or other keeping watch…

You might want to check this out.