The plague will be in Episode 8. There will be suspicions that it comes from Angband and is Morgoth's fault, but that will not be confirmed in any way on screen. The viewer can accept that explanation, or suspect that it might have something to do with new populations of Men entering Beleriand and (accidentally) spreading a new disease.
And even if Morgoth claims responsibility for killing Húrin's young daughter in Season 8, that would not disprove the alternative explanation.