In 2008, Ms. Guinness and her husband, Os, discovered that their townhouse in a suburb of Washington was contaminated with stachybotrys—also known as black mold. That launched a painstaking five-year renovation that cost as much as the couple originally paid for the house and involved ripping out walls, replacing many of the finishes and sterilizing nearly every surface and item they owned.

Jenny Guinness of McLean, Va., waited as men in moon suits cut away chunks of drywall in 2-foot increments. They would bag and seal the material, and start again. Soon they had removed the walls of an entire room. A mold remediator sat her down and said: “I have really bad news for you. This looks like it goes in every direction…I think you need to have a demolition company come in and start removing whole walls because I can’t see an end to it anywhere.”

In 2008, Ms. Guinness and her husband, Os, discovered that their townhouse in a suburb of Washington was contaminated with stachybotrys—also known as black mold. That launched a painstaking five-year renovation that cost as much as the couple originally paid for the house and involved ripping out walls, replacing many of the finishes and sterilizing nearly every surface and item they owned.

What they couldn’t clean, they threw away, including stuffed animals, sweaters and other items from their son’s childhood that they were saving for his kids. “I remember standing in the snow in the street in a moon suit, throwing these precious things into a bin,” Ms. Guinness says.

Almost every U.S. home has at least a little mold, but roughly 47% of homes have more substantial mold or dampness, says William Fisk, senior scientist at Lawrence Berkeley National Laboratory, who has researched mold for more than 10 years. People who are exposed to mold are 30% to 50% more at risk of asthma, coughing and wheezing, he adds. Mold exposure also has been associated with other health problems, such as bronchitis and respiratory infections.

Mold in homes hasn’t been tracked historically, but certain building practices—like making homes more airtight, installing air-conditioning units and using materials like drywall and oriented strand board, an engineered wood product—have made homes more conducive to mold growth than they were in the past, says Jeffrey May, principal scientist at May Indoor Air Investigations based in Tyngsborough, Mass.

Mr. Guinness, 71, a social scientist and great-great-grandson of Dublin brewer Arthur Guinness, has written or edited more than 30 books on subjects such as religion and politics. Ms. Guinness worked as a Vogue cover model under the name Windsor Elliott. She declines to discuss her age and is currently working on her memoir, tentatively entitled “Faces.” In 1997, right before their son C.J. entered college, the family downsized from a large home on 5 acres to a 3,000-square-foot townhouse in this well-to-do suburb whose residents include Newt Gingrich and Colin Powell.

In 2007, Ms. Guinness was diagnosed with lung cancer. Even after undergoing successful surgery, she still had trouble breathing in the house and, in early 2008, decided to test the home for mold. She discovered that black mold had streaked the firewalls and settled in thick clumps near the floor. The breadth of the mold shocked Ms. Guinness, who says she normally keeps her home spotless. “You could scrape it off with a spoon,” she says.

John Spangenberg, production manager of Columbia Restoration, a fire and water restoration service in Jessup, Md., broke the initial news to Ms. Guinness. He says the McLean townhouse had one of the worst cases of hidden mold he has seen in his 13-year career. “Most of the time when we deal with mold, you can usually find a stopping point. In her situation, the stopping point was after every wall was out,” he says.

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