Cohabiting with someone else can cause your bodies to start becoming more alike, a new study has found. In particular, the immune system seems to become more closely matched, possibly because you're sharing the same food, exercise regimes, sleep patterns, bathroom and (if you're really close) saliva as the people you're living with.

Researchers in Belgium studied the immune systems of 670 people over the course of three years. They found that married couples with children showed 50 percent less of a variation in their immune systems than randomly paired couples from similar demographics. Cohabiting but unmarried couples weren't included, but the team believes the results would follow similar trends.

In adults, about 25 percent of our immune system is dictated by genes, with the rest down to environmental factors. The aim of the study was to gain more understanding about how these environmental factors come into play, and that in turn could lead to benefits in the treatment of diseases such as diabetes and dementia.

We know that people in a relationship tend to develop similar habits, something called spousal concordance – a heavy drinker is likely to cut down on the booze when moving in with a teetotaller, for example. What Liston and his colleagues have shown is that these shared lifestyle habits, together with shared living conditions (levels of pollution and bacteria, for example) have a direct impact on the make-up of our immune systems.

The team also identified having children as an important factor. "It's not particularly nice to imagine, but the easiest way to transmit gut bacteria is the faeco-oral route – and parents could both be changing a baby's nappy," lead researcher Adrian Liston from the University of Leuven told New Scientist.

"That's at least something for prospective parents to consider," he added in a press release. "The sleep deprivation, stress, chronic infections and all the other challenges of parenting does more to our body than just gives us grey hairs. I think that any parents of a nursery- or school-age child can appreciate the effect a child has on your immune system!"

The study found that cohabiting and having children had a more significant effect on the immune system than infections such as flu or gastroenteritis, after which our internal biological landscape tends to return to its original form.

Gender didn't appear to have much of an influence on the immune system, but age did, and that in part explains why elderly people are more susceptible to infection and less responsive to vaccinations.

The team's work has been published in Nature Immunology.