The growth in rates of obesity and diabetes and the threat of antibiotic resistance could turn even a mild flu outbreak into a severe one, scientists have warned.

Scientists looked at what led to the 1918 Spanish flu, which spread rapidly around the globe as the First World War drew to a close, infecting a third of the world’s population and causing between 50 and 100 million deaths.

They say the world is far better prepared to tackle a flu outbreak today than in 1918 because of a better understanding of how flu spreads and improved surveillance of infections. But they warn that the rise of chronic diseases, coupled with changing population demographics and antibiotic resistance, could exacerbate an outbreak.

Scientists do not know when or how how big the next flu pandemic will be. “But we know that there will be one,” Dr Kirsty Short, virologist at the University of Queensland, told The Telegraph.

“As our population is ageing and chronic diseases are becoming so prevalent, that could turn even a mild pandemic into a chronic one,” she said.