I suspect that the worker got the virus on herself when taking her suit off.
She dresses up in the clean suit, goes in to tend to Duncan, gets virus on the suit, then gets it on/in herself somehow, while taking it off.
An alternative theory of mine is that the suit wasn’t fully decontaminated after the last use, and she got the virus on/in herself from the suit.
They aren’t deconning the suits. They’re tossing them after each use.
What they are NOT doing is covering their head/hair and having a bleach solution spraydown prior to taking off the suits.
Both of which are currently being done in the third world. Why can’t we do those simple things in our advanced healthcare system?