It’s not just living space, expenses and child-rearing tasks that you share with your spouse or partner – it’s also likely that you have similar immune systems.

Cohabitation and raising a child together has been found by European researchers to change a couple’s immune systems. The latter is in fact considered to have a greater effect on one’s immunity than traveler’s gastroenteritis or even the seasonal flu vaccine.

A team from the Babraham Institute in the United Kingdom and VIB and KU Leuven in Belgium examined the immune systems of 670 individuals ages 2 to 86, assessing factors such as age, gender, fitness level and, for the adults, whether they co-parented a kid or not.

Based on the results, those who lived and raised a child together had a 50 percent reduced variation in their immune systems, compared to the much greater diversity identified in the bigger population.

Immunologist and co-lead author Adrian Liston said their research is the first look at the immunity levels of two unrelated persons in a close personal relationship.

“It makes sense that [parenting] radically rewires the immune system - still, it was a surprise that having kids was a much more potent immune challenge than severe gastroenteritis,” says Liston, adding that there could be more to stress, sleep deprivation and other life changes than someone thinks parenting does to his or her system.

The team followed the subjects over a three-year period, regularly looking at their immune systems. Results showed that people tended to maintain stable immunity over time even with the intervention of the seasonal flu shot or gastroenteritis, and that immune systems display such elastic quality or bounce back after they are disrupted or challenged.

Age also emerged as an important factor for one’s state of immunity, consistent with declining response to vaccines and weakened resistance to infections seen in the elderly.

The findings were published in the journal Nature Immunology.

It only makes sense that living with a partner would significantly affect one’s body and health, given shared routines, synchronized sleep schedules and exposure to the same pathogens or pollutants. Kissing and other forms of physical contact, too, allow the exchange of both good and bad bacteria.

In 2013, a study found that families living together shared a more similar gut bacterial profile.