Woolly Adelgid is an insect not a virus.  These are easily controlled with an application of Bonide Annual Tree & Shrub Insect Control.  This product will be diluted with water and poured around the base of the hemlock.  It will be taken up through the roots and enter every part of the plant.  When the adelgids suck sap from the hemlock, they get the insecticide too and are killed… they can’t escape.  Do this right away while the tree is actively growing.  How much you use depends on the size of the tree so read and follow the direction to the letter.