The trillions of microorganisms that live in and on our bodies – known as the microbiome – have been hailed as the key to better immunity. “Lots of studies correlate the types of bacteria in your microbiome with health and disease across almost every mental and physical condition,” says Prof Daniel M Davis, head of life sciences at Imperial College London and the author of Self Defence: A Myth-busting Guide to Immune Health. “But most of that evidence is correlative, and we still need to understand exactly how the microbiome affects health.”

Scientists often look at one measure: diversity. In other words, how many different species of microbes live in the gut. “The more diverse your microbiome is, the more it seems to correlate with not being ill.”

However, studies indicate that the story is more complicated.

“New research suggests that what really matters is competition,” says Davis. “If groups of bacteria are feeding off each other and synergising in ways that help them multiply rapidly, that can be bad for you.” This could contribute to diseases occurring, or symptoms being worse. “But if they’re competing with one another for resources, that tends to keep things under control. A microbiome with lots of competing species can help maintain balance.”

So what should nonscientists take from this? “Yes, the microbiome is important and it influences the immune system’s ability to fight infection,” Davis says. “But we still don’t fully understand what makes a ‘healthy’ microbiome. We don’t yet know how to reliably manipulate it to make people healthier. For now, that means being cautious about bold commercial claims. “Be very sceptical of products that promise ‘this is going to change your microbiome and make it healthier.’ The science just isn’t really there yet.”