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More than half a million Americans will develop Alzheimer's disease this year, but as many as half will never be told their diagnosis, according to a new report.

Doctors are reluctant to give the bad news, are afraid of the reaction, or fear they won't be believed, the Alzheimer's Association says. But Alzheimer's patients and their caregivers say they want to know.

"I think to have a diagnosis gives you at least a place to start," says Mary Downs of Reston, Virginia. Downs should know — she cares for her mother-in-law Helen, who at 83 has a diagnosis, and for her own mother, Lois, who has some symptoms but has not been diagnosed.

"There are some things with this disease we can control," Downs told NBC News. "We can have a plan for helping the person with a disease. When Helen got the diagnosis, we had a chance as a family to say, 'We know she has this.' It kind of gave us the chance to sit down and ask, 'What do the next years with her look like?'"

In its annual report on Alzheimer's dementia, the Alzheimer's Association finds that 5.3 million Americans have the disease, including 200,000 people under the age of 65. "Barring the development of medical breakthroughs, the number will rise to 13.8 million by 2050," the association says in its annual report. Two-thirds of them are women.

This year, the organization looked at who gets an actual diagnosis. It's not straightforward — there is not a blood test, for instance. But a trained clinician — a doctor, nurse or other expert — can diagnose dementia with a series of pencil-and-paper tests.

In their survey, only 45 percent of people with Alzheimer's disease or their caregivers said they were given a diagnosis by their doctor.

It's not always clear whether those numbers are truly precise — most Alzheimer's patients have a batch of other health problems, such as high blood pressure or diabetes. That might be the diagnosis that gets written down and billed for.

But the organization notes that more than 90 percent of people with the four most common cancers — breast cancer, colon cancer, lung cancer and prostate cancer — got a clear, verbal diagnosis.

"These disturbingly low disclosure rates in Alzheimer's disease are reminiscent of rates seen for cancer in the 1950s and '60s, when even mention of the word 'cancer' was taboo," said Beth Kallmyer, vice president of constituent services for the Alzheimer's Association.

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"It rings true with what we hear doctors saying," she added. "It also rings true with what we hear from families in helplines. They say, 'We don't understand — we didn't get a diagnosis of Alzheimer's'. We think it is critical that people get that information, because if they don't get that information they miss out on making important decisions in their lives."

Helen Downs' diagnosis gave the family an opportunity to get her moved into their home, where Mary looks after her full-time.

"We had a chance to ask, 'Where do you want to live?'"

Helen made it clear she wanted to stay with family for as long as possible.

Mary Downs has doubts about her own mother, Lois, who is 87 and lives nearby. "I think she shows some dementia, but she does very well," Downs said. "She's very independent, and it's difficult to get her to see that there is anything wrong." A diagnosis would provide an opportunity to have a good, hard talk.

"We could say, 'Mom we have to think about these things even if you don't want to accept them,'" Downs said. "Being able to plan is what is important."

It's true that the diagnosis is devastating. There is no cure for Alzheimer's and not even any good treatment, although companies are working on it. And there are steps people can take to prevent it if they are at high risk and people can remain functional for years if they are diagnosed early.

Cynthia Guzman was ready to accept her own diagnosis. Now 66, Guzman guessed something was wrong when she got lost driving.

Cynthia Guzman was ready to accept her own diagnosis. Now 66, Guzman guess something was wrong when she got lost driving. Alzheimer’s Association

I drove 300 or 400 miles a week, and I never got lost. I had a pretty good memory," Guzman, a retired nurse living in Napa, California, told NBC News.

"I was at a stop sign. I didn't know where I was going or how I got there."

Testing showed she has early onset Alzheimer's. "I was glad to have a diagnosis," she said.

Kallmyer says doctors need to know this.

"At some point, you can't deny it."

"In the early stages, people can still talk to family members about what type of care they want. And they can participate in clinical trials. Not having that information robs them of the opportunity to make those decisions," Kallmyer said.

The organization is leading a lobbying effort in Washington, D.C., later this week to support legislation that would encourage and allow doctors to bill Medicare for time spent counseling not just patients, but also their caregivers.

"Everybody understands that doctors are under enormous pressure to do a lot in a short period of time, and talking about Alzheimer's disease takes time," Kallmyer said.

The Alzheimer's Association says treating dementia this year will cost the U.S. $226 billion, of which $153 billion is the cost to Medicare and Medicaid alone.

"In 2014, the 15.7 million family and other unpaid caregivers of people with Alzheimer's disease and other dementias provided an estimated 17.9 billion hours of unpaid care, a contribution to the nation valued at $217.7 billion (with care valued at $12.17 per hour)," the association says.