The $43.5bn trust is the largest philanthropic organisation in the world and is committed to the eradication of malaria and polio, and controlling the spread of tuberculosis and HIV

The story of the world’s largest private foundation begins with a couple reading a news article about suffering in the developing world. The husband clipped the article and sent it to his father, the philanthropist William H Gates, writing: “Dad, maybe we can do something about this.”

It was the first step towards the creation of the Bill and Melinda Gates Foundation. Since beginning their great philanthropic mission in 1997, the husband and wife team have become a powerful catalyst for the improvement of lives in the world’s poorest countries.

The foundation’s trust endowment of $43.5bn (£29.5bn) makes grant payments in excess of $3bn every year ($3.9bn in 2014). Its focus has been on bridging the enormous health deficit between rich and poor countries and on fights it sees as vast, but ultimately winnable. Among its goals are the eradication of malaria and polio, and controlling the spread of tuberculosis and HIV.

Investor Warren Buffet joined the foundation as a trustee in 2006 with a £30bn pledge. In 2008, Bill Gates resigned as chief software architect at Microsoft, the technology company he built, to focus solely on the work of the foundation.

In total, the fund has given $32.9bn in grants to health programmes around the world. Its work focusses on prevention, immunisation and vaccination.

Since the turn of the century, partly thanks to the work of the foundation, four countries have eradicated malaria. Mortality from the disease has dropped 42% in that time. Bill said in 2014 that malaria could be eliminated within a generation.

In 2014, after a massive coordinated effort between the Indian government, the Gates Foundation and Rotary International, India announced it was officially polio-free. The programme employed 2m vaccinators who spread out across the country. Just five years before, India had more than half the world’s polio cases. It was “the greatest global health achievement I have ever witnessed”, said Bill. The foundation now aims to eradicate polio worldwide by 2018.

The foundation has also funded the Guardian’s award-winning Global Development website since 2010.

As well as his work on health through the foundation, Bill has made climate change and clean energy a personal mission. The foundation’s annual letter for 2015 says their achievements could be undone by the advancing climate crisis.

“It is fair to ask whether the progress we’re predicting will be stifled by climate change. The most dramatic problems caused by climate change are more than 15 years away, but the long-term threat is so serious that the world needs to move much more aggressively – right now – to develop energy sources that are cheaper, can deliver on demand, and emit zero carbon dioxide. Bill is investing time in this work personally (not through our foundation) and will continue to speak out about it.”