Alzheimer's disease is the third-leading cause of death in the U.S. after heart disease and cancer, according to a new study conducted by the Rush Alzheimer's Disease Center in Chicago.

The study, published Wednesday in the journal of the American Academy of Neurology, challenges other major health organizations that do not place as much emphasis on the disease as a leading cause of death.

According to the Centers for Disease Control and Prevention, Alzheimer's is the sixth-leading cause of death. The underreported severity of the disease is linked to the fact that death certificates are a primarily used source for finding the cause of death, The Washington Post reported.

"A larger number of deaths are attributable to AD dementia in the United States each year than the number reported on death certificates," the study's authors said.

Researchers kept track of 2,566 people ages 65 and older for a period of eight years. Every year the subjects were tested for Alzheimer's-type dementia. Researchers also noted the chances of death in those who received a clinical diagnosis and those who did not, The Washington Post reported. 

"Death certificates may not be the best way to measure how many people die from something that takes up to 10 years" epidemiologist Bryan James, the study's lead author, told The Washington Post.

Death from Alzheimer's, a memory-loss disease that affects the elderly, is not always listed as the cause of death because the person may not die directly from it. If a person with Alzheimer's loses the ability to swallow, that can result in food going into the windpipe, The Washington Post reported. That person's chance of dying of pneumonia is then increased.

 "We're not saying they didn't die of those things; we're just saying, 'Well, what put them in the hospital with that condition?' " James said.

The number of people diagnosed with Alzheimer's, which is always fatal, has skyrocketed in recent years. An estimated 5 million Americans have the disease, a number that is expected to triple in size by 2050, according to the Alzheimer's Association, The Washington Post reported.

Experts in the disease suggest a way to reverse the underreported deaths is to increase funding for Alzheimer's, which does not receive as much funding compared to cancer.

"There are many people who still think of Alzheimer's disease as just a memory problem- you forgot where you left your keys," Keith Fargo, director of Scientific Programs and Outreach at the Alzheimer's Association, told The Washington Post. "But currently, Alzheimer's is a universally fatal brain disease that has kind of fallen by the radar."