Microsoft founder Bill Gates has pledged $100 million of his own wealth to Alzheimer's research, dividing the money between startup ventures and an established private-public research partnership.

"It's a huge problem, a growing problem, and the scale of the tragedy -- even for the people who stay alive -- is very high," Mr. Gates told Reuters in an interview.

Of Mr. Gates' personal contribution, $50 million will go to the Dementia Discovery Fund, a partnership between charity, pharmaceutical companies and governments working to advance Alzheimer's research.

The other $50 million will go to startup companies pursuing "less mainstream" approaches to treating the disease, although the companies have not been chosen yet, Reuters reported.

In a blog post, Mr. Gates outlined his reasons for donating, saying that Alzheimer's and dementia greatly reduce quality of life, take an extreme emotional toll on families and put an immense economic burden on the nation's health care system.

"I know how awful it is to watch people you love struggle as the disease robs them of their mental capacity, and there is nothing you can do about it," he wrote. "It feels a lot like you're experiencing a gradual death of the person that you knew."

At least 5 million people in the U.S. are living with Alzheimer's and that number could grow to 16 million by 2050, according to the Alzheimer's Association.

It is the sixth-leading cause of death in the U.S. and there is no cure, with current medications only treating symptoms.

The economic burden is estimated at $259 billion annually, according to the Alzheimer's Association, and more than 15 million Americans provide unpaid care for people with Alzheimer's and other forms of dementia.

Mr. Gates said in his blog post that while men in his family suffer from Alzheimer's, his personal connection to the disease is not the only reason for the investment.

He identified key areas that his investment aims to benefit: understanding how Alzheimer's develops, improving detection and diagnosis, more approaches to stopping the disease, increase access for patients to clinical trials and more availability of established data.

"This is a frontier where we can dramatically improve human life," Mr. Gates wrote. "It's a miracle that people are living so much longer, but longer life expectancies alone are not enough. People should be able to enjoy their later years -- and we need a breakthrough in Alzheimer's to fulfill that. I'm excited to join the fight and can't wait to see what happens next."