A 22-year-old violinist who was shot in the arm and head during last week's theater shooting in Aurora, Colo., is expected make a full recovery, doctors say, in part because of a brain condition she didn't know she had.

[COMPLETE COVERAGE: Colorado theater shooting]

Petra Anderson sustained multiple gunshot wounds, including a bullet that entered her brain through her nose. But miraculously the bullet "traveled through Petra's brain without hitting any significant brain areas," Brad Strait, a senior pastor at Cherry Creek Presbyterian Church in Englewood, Colo.—Anderson's church—wrote in a blog post after spending a day with her in the ICU:

The doctor explains that Petra's brain has had from birth a small "defect" in it. It is a tiny channel of fluid running through her skull, like a tiny vein through marble, or a small hole in an oak board, winding from front to rear. Only a CAT scan would catch it, and Petra would have never noticed it. But in Petra's case, the shotgun buck shot, maybe even the size used for deer hunting, enters her brain from the exact point of this defect. Like a marble through a small tube, the defect channels the bullet from Petra's nose through her brain. It turns slightly several times, and comes to rest at the rear of her brain. And in the process, the bullet misses all the vital areas of the brain. In many ways, it almost misses the brain itself. Like a giant BB though a straw created in Petra's brain before she was born, it follows the route of the defect. It is channeled in the least harmful way. A millimeter in any direction and the channel is missed. The brain is destroyed. Evil wins a round.

Anderson has been removed from the ICU, Strait said on Wednesday, and her condition continues to improve.

"There is much ahead," he wrote. "More surgeries. Facial reconstruction, perhaps."

More than $164,000 has been already raised to help cover Anderson's medical costs. The Colorado native graduated from the University of the Pacific in May with a bachelor's degree in music composition, and had been accepted into the graduate program at the University of Maryland School of Music. She did not have health insurance.

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[Also read: Donors give nearly $2 million to help Aurora shooting victims]

Strait's blog post about Anderson, meanwhile, appears to have gone viral, averaging more than 71,000 page views per hour, he says:

This means that the comments traffic has been overwhelming, and beyond our ability to keep up with. By far, most comments have been wonderful and supportive. God bless you all. All kinds of ideas and thoughts have been posted: thankful, angry, sad, questioning, amazed, rude, kind, and insightful. The world is filled with people who see this tragic event from different perspectives. I am thankful for all of you who commented, whether you agreed with my world view and faith perspective or not. We have been forced to think and reflect—a very good thing. I hope the dialogue continues in other venues.

There's a sad footnote to this survivor story, however. According to the pastor and the campaign's website, Anderson's mother, Kim Anderson, has terminal cancer.