Should Caesarean-section babies be smeared with a sample of their mother's vaginal fluids as soon as they are born?

"Vaginal seeding" is not mainstream medicine, but it is growing in popularity.

The idea is to give these newborns something they missed when they emerged into the world - the good bacteria that live in their mother's vagina.

A swab is taken of mum's vaginal fluid, which is then rubbed on to her child's skin and mouth.

The hope is this microbial gift will boost their child's long-term health - particularly by reducing their risk of immune disorders.

It is a crucial time.

We might have been sterile in the womb, but in our first few moments of life an invisible bond is being established between baby and bacteria.

It's a relationship that will last a lifetime, and the first contact is as important as a first date.

"The first time a baby's own immune system has to respond are to those first few bacteria," says Prof Peter Brocklehurst, from the University of Birmingham.

"That we believe is important for, in some way, setting the baby's immune system."

There is a noticeable difference between the microbiomes - the collection of bacteria, viruses, fungi and archaea - of babies born vaginally and by Caesarean section.

It lasts for about the first year of life.

A baby born vaginally is first exposed and colonised by microbes from their mother's vagina and gut.

But for Caesarean-section babies, the first exposure "if they're lucky", says Prof Brocklehurst, comes from the very different organisms on their mother's skin.

He is running the Baby Biome Study to see if these different microbial colonists on Caesarean-section babies explain why they have higher rates of diseases such as asthma and allergies later in life.

The early interaction between the immune system and microbes appears crucial.

Obviously our bodies do attack the dangerous ones - but the overall relationship between microbial and immune cells is about more than conflict, it's a far deeper dynamic.

Graham Rook, a professor of medical microbiology at University College London, says the microbiome is the immune system's teacher.

"This is a learning system, it is like the brain. Now, the thing about the adaptive immune system is it needs data, just like the brain needs data."