Microsoft co-founder Bill Gates will donate $50 million (£38.1 million) to the Dementia Discovery Fund and a further $50 million towards outside-of-the-box startups to help fight Alzheimer's disease.

Worth an estimated $90 billion, Gates will donate the money separate from the Bill & Melinda Gates Foundation - which has donated around $44 billion by the end of 2014. In August, Gates donated another $4.6 billion in his biggest single donation since 2000.

Speaking to Reuters, Gates said he wanted to help combat the rapidly rising numbers of Alzheimer's sufferers. "It's a huge problem, a growing problem, and the scale of the tragedy - even for the people who stay alive - is very high," he said.

Gates was optimistic that with the right funding and innovation, better treatments could be discovered.

"It'll take probably 10 years before new theories are tried enough times to give them a high chance of success," Gates said. "So it's very hard to hazard a guess (when an effective drug might be developed). I hope that in the next 10 years that we have some powerful drugs, but it's possible that won't be achieved."

Gates has pledged to donate half of his wealth as part of the Giving Pledge, which was started by himself and Warren Buffett. The pledge asks billionaires to give away at least half of their wealth to charity. Tim Cook, Elon Musk and Richard Branson are among 158 individuals who have signed the Giving Pledge.

Gates has not yet identified where he will donate the extra $50 million. "I know how awful it is to watch people you love struggle as the disease robs them of their mental capacity," he said. "It feels a lot like you're experiencing a gradual death of the person that you knew."

According to Gates, there are five areas that need to be addressed:

  • Understanding better how Alzheimer's unfolds.
  • Detecting and diagnosing early.
  • Finding different paths to stop the disease.
  • Making it easier for people to join clinical trials of new drugs.
  • Using data and information better.