Learn Swedish: Eftersom viruset inte försvinner är jag stressad. - Because the virus does not disappear, I am stressed.

This is the BIFF rule in action! Because eftersom (because) starts a subordinate clause, the negation inte must go before the verb försvinner. If this were a simple main clause, it would be Viruset försvinner inte.

Why is it är jag instead of jag är in the second half?

This happens because of the V2 (verb-second) rule. The entire subordinate clause (Eftersom viruset inte försvinner) counts as the first block of information. The main verb (är) must be in the second position, so it swaps places with the subject (jag).

Is virus an en-word or an ett-word?

It is an ett-word: ett virus. To say 'the virus', you add the definite ending -et to get viruset.

Could I use därför att instead of eftersom to mean 'because'?

In Swedish, you generally do not start a sentence with därför att. When 'because' is the very first word of the whole sentence, you should almost always use eftersom.