When Hagarty began noticing weakness in her right leg in 2007, doctors thought the problem was associated with a herniated disk. Three back surgeries followed between December 2007 and June 2009, accompanied by physical therapy, but the nurse kept getting weaker.
At the Hines VA hospital, where Hagarty specialized in helping people with breathing problems, she used a scooter and kept a brave face.
"We knew she was having a hard time walking, but we thought it was complications from her surgeries," said Patricia Moss, Hagarty's supervisor and associate chief nurse for long-term care.
Hagarty had worked at the medical center for almost 39 years, earning deep respect from colleagues for her discipline, commitment and professionalism.
"Anything she took on, she went at it 150 percent," Moss said, adding, "Everyone was shocked when she left."
Hagarty told no one at Hines of her ALS diagnosis and declined a goodbye dinner after leaving the job she loved. Months of depression followed as she was overcome by feelings of loss. Her sister and her family arranged rotating caregiving duties, but tempers flared under the stress of Hagarty's deterioration and their sense of helplessness.
All Hagarty could think of was patients she'd treated at Hines who were "just eyeballs looking out at you with everything else every function except their awareness and their feelings gone."
Things began to shift only when her sister pleaded with Hagarty to return to the moment and the people who loved her.
"Focus on what you have on the good you can still do," Rita Risatti remembers saying.
The sisters had lost their mother to cancer when they were young, and Hagarty realized she didn't want to go as her mother had, without any record of her thoughts and feelings left behind. Soon, she began to write and shop over the Internet, planning the gifts and memories she would give her family and getting intimately involved in her medical care.
Acceptance dawned and, with it, the desire to let people know what she was going through. Hagarty had not told anyone outside her closest family of her illness, but last Monday she announced it to 60 friends, cousins and former co-workers in a Christmas letter.
"I guess I don't want my last message to people to be (that) I denied the whole thing," she said.
"Eileen is extremely resilient and courageous, and her emotional journey has been just amazing," said Nicole Sammartino, patient services coordinator for the ALS Association Greater Chicago Chapter.
"As a nurse, Eileen tried to help her patients focus on the positive aspects of living with their diseases. Now she's trying to practice what she preached," said Mary Schick, her case manager from Rainbow Hospice.
As for challenges that lie ahead, Hagarty is characteristically decisive, motivated by a desire to spare her family and herself undue suffering.
Terrified of being unable to breathe, of choking, she has told her husband and her children she does not want to be placed on a ventilator.
"Absolutely not; I don't want to live like that," she asserted recently when a visitor came to her home.
More than 85 percent of ALS patients similarly decide to forgo mechanical ventilation, said Rowin, Hagarty's doctor, acknowledging that's a rough estimate.
Jon Hagarty knows he hasn't come to terms with that decision.
"I'm reconciled to it because it's what she wants, but when push comes to shove, maybe she'll change her mind," he said, sitting in his living room next to his wife.
Hagarty's daughter said it tears her apart to imagine what her mother's decision means.
"When she got her 'do not resuscitate' papers ready, it almost killed me because I'm thinking: What if she's dying in front of me and I'm not able to do anything?" the young woman said, breaking into tears.
Son Patrick said he wants his mom to hold on as long as she can.
"As long as she is able to pull even a little bit of joy out of life, as long as you see a smile on her face, then life is worth living," he said.
As for Hagarty, she tries not to think too hard about the future, filled with darkness. Instead, she counts her blessings today.
"The wonderful caregiver that somehow I was so lucky to find," she said. "Financial resources that have allowed me not to go bankrupt. Our new van that lets me get out of the house. My hospice people, they're wonderful; my nurse, she's an angel. And the love and support of my family and my sister, above all else."