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“I guess you really don’t know how much I love you and miss you,” the 16-year-old girl wrote her family in St. Peter, Minn., from the tuberculosis sanatorium where she had been placed, 225 miles north in the pine forests near Walker.

“Life has certainly been miserable these past 3 days for me being stuck away up here away from all my loved ones. Might as well be 300,000 miles away as 300. Oh mommy, why oh why did I ever have to get such a thing? Guess I’m not such a grown-up person after all. Just a very homesick little girl aching for her family’s affection and love.”

Now nearly 91, Marilyn Barnes Robertz dabs tears over the painful memories rekindled nearly 75 years later as she rereads her letter from Oct. 31, 1943.

She wrote it on the third day of a nearly three-year stay at the tuberculosis treatment center known since 1922 as Ah-Gwah-Ching, an Ojibwe phrase for “out-of-doors.” It sounded better than the original 1907 name: the Minnesota State Sanatorium for Consumptives.

A disease packing a dry cough, bloody sputum and recurring fevers, TB claimed more than 20,000 lives in Minnesota between 1887 and 1899. That prompted the state to open more than 20 sanatoriums to quarantine the sick and enforce complete bed rest. Robertz was just one of 14,000 patients to go through Ah-Gwah-Ching and one of 50,000 kept in sanatoriums.

Now her old letters have been compiled into a book published this month by the Minnesota Historical Society Press, “The Girl in Building C: The True Story of a Teenage Tuberculosis Patient.” The 224-page book offers a glimpse, both horrifying and heartwarming, into the era before antibiotics began countering the TB scourge in the 1940s.

Mary Krugerud, a tuberculosis expert from Hutchinson, Minn., wrote an earlier book about the disease in Minnesota and landed a Minnesota Historical Society fellowship to expand her TB research a few years ago.

She had hoped for something personal, perhaps a diary. What she found was a stack of 300 letters that Robertz had penned in the 1940s.

“I hoped to connect with someone who remembered this delightful person and could tell me more about her,” Krugerud writes in the book’s introduction. She instead discovered that Robertz was alive and well in St. Peter, where her great-grandparents from Norway and Germany settled around the time of statehood in 1858.

Reluctant at first to revisit the letters, Marilyn agreed to team up with Krugerud. They worked for a year on the book, which bounces between her old letters and Krugerud’s informational notes about the disease and the lives it wrenched.

“It’s a pure joy for me to know that people find this history interesting,” Robertz said. “It shows how we had to fight this terrible disease and how letters save history.”

Both women worry about the all-but-lost art of letter writing in the digital age.

“Who will donate letter collections in the future, with everyone resorting to Facebook and Skype to keep in touch?” said Krugerud. “Most other tuberculosis books are based on diaries or memoirs written though the fog of intervening years.”

Robertz’s mother, Virginia Barnes, saved the thick pile of letters and postcards. Coming “from an observant teenager who wrote well,” Krugerud said they “are probably the truest documents of pre-antibiotic sanatorium life in existence.”

In her first letters home, the young Marilyn brags about fresh bananas and lemon pie — rarities during World War II rationing. Nevertheless, her weight dropped to 103, down from her usual 125 pounds.

Letters chronicle surgery on her ribs to create more room for her failing left lung. She also writes about the death of friends: “Ralph passed away … it was really a blessing he did go — poor kid. He was only 22 yrs old.”

She was particularly smitten with another boy, Maurice “Benny” Bensen, who died in 1945 after surgery.

“Benny & Ralph — both such swell boys, too. I guess I won’t have any more so-called crushes while I’m up here — will wait til I get home and can pick some good healthy fella — huh, Mom?”

The daughter of a letter carrier, Marilyn received limited visits from her family because her dad couldn’t get off work and gas shortages required a daylong Greyhound ride.

“I learned to be patient and assertive,” she said in a recent interview. “I had to speak up for myself with no family around and never let anyone walk over me. At the same time, I tried to be kind and not to be in their face.”

She was healthy enough for release in August 1946, returning to high school as an older junior. From there, she went to Gustavus Adolphus College in St. Peter, where she found her “healthy fella”: Bill Robertz, who went on to become a speech teacher at Gustavus. They married in 1953 and raised two kids.

Two weeks ago, Marilyn and Bill celebrated their 65th anniversary with wine, carrot cake and her favorite plant — coleus — on every table at their assisted living home.

Curt Brown’s tales about Minnesota’s history appear each Sunday. Readers can send him ideas at mnhistory@startribune.com. His new book looks at 1918 Minnesota, when flu, war and fires converged: https://tinyurl.com/MN1918. Podcasts at www.onminnesotahistory.com.

‘The Girl in Building C’

“Oh mommy, why oh why did I ever have to get such a thing? Guess I’m not such a grown-up person after all. Just a very homesick little girl aching for her family’s affection and love.” – Marilyn Barnes’ letter home to St. Peter from a TB sanatorium, Oct. 31, 1943 — the same month she turned 16.

Book event: Mary Krugerud will discuss and read from “The Girl in Building C,” Sept. 26, 7 p.m. at Excelsior Bay Books, 36 Water St., Excelsior.