Arguments about washing the dishes isn't the only thing you share with your roommates. You also share much of your immune system.
A new study published in Nature describes how researchers created a "cellular immune profile" for 670 people. They found that each person had a long-term baseline immune system profile, which didn't shift despite short-term changes when a subject got sick or got a vaccine. The only thing that shifted this baseline in the long term was living with another person.
"The largest influence on immunological variation identified was cohabitation," says the study, "with 50% less immunological variation between individuals who share an environment (as parents) than between people in the wider population."
The "parents" part of that sentence is important, as childless partners or roommates weren't compared in this study, so you have to take the germy filth of children into account.
The study actually set out to see how an individual's immune system varies over three years. It showed remarkably little change in the subjects' immune profiles. But when researchers compared the profiles of subjects in order to see how much we all vary, they found something rather surprising—parents living together were more similar to each other than to anyone else.
The idea is that people who live together develop a resistance to a shared environment. Because the researchers didn't set out to explore this aspect, according to an article in Nautilus, some conjecture is involved, but they think that exposure to similar microbes forces the parents' immune cells to develop similar defenses over time. And their children? That might contribute simply by bringing more dirt into the home.
"For example, children are likely to increase the exchange of gut bacteria by reducing the sterility of the household," says lead author Adrian Liston, from the University of Leuven in Belgium.
Further research is needed to see if this effect is seen in childless households, but it could be that you're sharing a lot more with your partner than you previously thought.