People who develop Alzheimer’s disease typically experience up to six years of accelerated mental decline before the disease presents itself, according to new research.

For their study, researchers at the Rush Alzheimer’s Disease Center in Chicago followed 2,071 older adults without dementia who participated in two separate studies for 16 years. Over that time, 462 people developed Alzheimer’s disease. During the six years prior to developing Alzheimer’s disease, these participants experienced rapidly accelerating cognitive decline at a rate of about fifteen-fold, researchers noted. This severe decline was not present in individuals who did not develop Alzheimer’s, according to the research, which appears in the March edition of the Archives of Neurology.

A separate report released Tuesday found that nearly 15 million Americans are caring for a loved one with Alzheimer’s disease. The Alzheimer’s Association, which published its report in the journal Alzheimer’s & Dementia, said that 14.9 million caregivers provide 17 billion hours of unpaid care. This care is valued at more than $200 billion per year, researchers calculated. There are currently about 5.4 million people in the U.S. suffering from Alzheimer’s disease, according to the report.