Your immune response to a flu virus stems from a combination of the viruses you've caught in the past, the house you grew up in, and your genes. A 2016 study in the journal Nature Immunology found that our immune systems are largely shaped by our age and the environments we've lived in. Because of that, some people won't be susceptible their partner's flu at all, and others will. You can't change your past.
It's also worth remembering that if your partner, child, parent, or roommate is already showing some common symptoms of the flu — like fever, chills, congestion, fatigue, or cough — it may be too late to protect yourself. Scientists say influenza can incubate in the body for one to four days before a person shows any signs of harboring the virus.
That means your partner can be contagious and not know they're spreading the illness yet, as the CDC points out.
The best thing to do is keep exercising, sleeping, seeing your friends, and eating healthy food, which are all activities that will help your immune system stay strong.
Hopefully your "other half" will be able to join you in those activities again soon, in good health. The flu typically lasts about five to seven days.