When people think about the biodiversity crisis, images of rainforests being bulldozed and species going extinct probably come to mind, but in recent months, I’ve been exploring a much smaller biodiversity crisis – the one inside us.

First, this week’s most important climate headlines – and a reminder that the pivotal Cop30 conference kicks off in Belém, Brazil, next week. The Guardian will have unrivalled coverage from our team of reporters on the ground, some of whom you’ll hear from in extra editions of Down to Earth over the next two weeks.

Essential reads

‘New reality’: Hurricane Melissa strength multiplied by climate crisis, study says

Still a chance to return to 1.5C climate goal, researchers say

Life and death on India’s toxic trash mountains – video

In focus

Trillions of bacteria live in and on the human body. Illustration: Pete Gamlen

Each human body is home to its own miniature ecosystem, made up of trillions of bacteria, as well as smaller communities of fungi and viruses. They live in a community in the same way animals, trees, water and rocks live together in a forest. These microscopic worlds are known as the human microbiome.

Picturing microbes all over our bodies might sound unnerving, but these ecosystems help us stay healthy – just like we need to preserve ecosystems in nature, we need to preserve them in our body, too.

For Tim Spector fans, it’s not all about the gut. Microbes in the vagina keep it acidic and protect it from invading bacteria, while those on the skin are specialists in wound healing. More than 100 species of fungi live on the feet alone – every community is different.

Dr James Kinross, a reader in surgery at Imperial College London and author of Dark Matter: The New Science of the Microbiome, describes the loss of microbial biodiversity inside our bodies as an “internal climate crisis”. Microbiomes are being destroyed by the misuse of medicines – particularly antibiotics – by ultra-processed foods and by the loss of contact with a healthy natural environment.

“There are very simple and important things that you can do to change it,” Kinross says. “And it shifts the emphasis away from more traditional ways of medical thinking about our health – ie, we’ve got to just kill all microbes dead, and that way we’re going to be healthy – to a more holistic strategy that reconnects us with nature and has at its heart a kind of conservation strategy.”

A growing body of research is linking two layers of biodiversity. There is the outer layer of biodiversity (soil, water, plants, animals, etc), and then there is the inner layer: the biodiversity that lives within and upon the human body. Our bodies evolved to exchange microscopic bacteria, viruses and fungi with the environment that surrounds us.

Kinross says the study of the microbiome is becoming a more credible science. “I spent 20 years going around the world speaking to very empty lecture rooms. Now, those lecture halls are starting to fill out,” he says.

Urbanisation isolates people from nature, reducing time spent in natural spaces, inhaling, touching and ingesting beneficial microorganisms. Planners are exploring how cities can help reconnect people with nature – bringing flowers, healthy water bodies and old trees into our cities.

Finland is leading the way on this with an impressive programme to boost nature contact in kindergartens. Across the country, 43 daycare centres have been awarded a total of €1m (£880,000) to rewild their yards and increase children’s exposure to microscopic biodiversity.

The kindergarten I visited outside Helsinki had literally imported a forest floor into the play area (which used to be an old car park). I made mud pies with five-year-olds – and spoke to some scientists – to hear about how this radical experiment had transformed children’s health.

But you don’t need to move a forest to enhance your microbiome.

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An amazing study from Finland showed that just one month of growing plants boosted skin bacteria and the body’s immune response. It was not the act of gardening that was important, but contact with healthy, biodiverse soils.

Green walls installed inside offices can diversify the skin ecosystems of people working there. A study on a green wall created by a Finnish company, Naava, which uses a patented system of air circulation that draws air through the root of the plant, found that in just two weeks there was an increase in the relative abundance of Lactobacillus bacteria on the skin of employees, which has been found to prevent skin infections.

Another study found that having pets could be good for human health. To maximise health benefits, owners should allow their pets regular access to the outdoors. Bed dust is also no bad thing, researchers found.

Humans are part of nature and covered in millions of beneficial bacteria that should be shared, says Kinross. People with larger social networks tend to have more diverse gut microbiomes. For example, teenagers have more diverse and resilient microbiomes, and similarly, when an elderly person goes into a care home, their microbiome diversity increases.

“In urban environments we’re so isolated. We live on top of each other, but really we live in a very disconnected way,” says Kinross, who recommends eating with other people as a way to share microbiomes. “Each house, each room, has its own microbiome. At a friend’s house you’re getting exposure to all the environmental microbes that are within – on their cutlery, on their plates, in their kitchen.” Meaningfully connecting with other people “is important for all measures of our health, not just the microbiome”, he adds.

After the colon, the mouth is believed to be the most diverse bodily ecosystem. Eighty million bacteria are transferred in a kiss. “Kissing is good, you should do that for your microbiome,” he says. To get the benefits, “you need to have a good old snog”. Perhaps a reason to get the mistletoe up early this year.

Read more:

The human microbiome: why our microbes could be key to our health

The brain microbiome: could understanding it help prevent dementia?

How we’re killing our microbiome and kimchi alone won’t save it – video