I'd be careful with this one. For a lot of people, "I'm interested in global health" means "I want to go on a mission trip for two weeks once in my life and call myself a humanitarian." I'm not saying this is you at all, but I think many faculty - especially those with extensive experience in global health - may be wary of this claim if you don't have the experience to back it up. They will be skeptical because they've probably seen many students claim this same interest over the years to use as a talking point to get into good schools, only to never follow up on that vague interest later in their careers.
If you really must write about it, I agree with
LizzyM's advice - tie it in to the skills you already have. When have you had to work in a setting with limited resources? Maybe you volunteered at an underserved school/federal public health clinic/etc? Have you spent any time learning foreign languages? Have you had to forge an alliance with a community that was very distrustful of you at first?
If you haven't done any of these things, I recommend gaining those skills before deciding if a career in global health is what you really want. It's a sexy topic these days, but it is way way harder than it's made out to be in the documentaries and such. Imagine watching a patient bleed in childbirth and having no blood available to transfuse them with. Imagine telling your hundredth patient with osteoarthritis that you don't know when you can perform their knee replacement because there are no funds to buy the implants, and no one is donating the parts. Imagine having a 19-year-old mother bring in her barely arousable child to ask you what's wrong, only to figure out the problem is that she's feeding her kid Pepsi because she's too malnourished to produce her own milk.
I worked in Central America for a few years before medical school, and these are all true scenarios I encountered. Global health is amazing, and it's 100% worth it, but it's not for the faint of heart. I recommend gaining some real experience in the field before deciding it's what you want to do forever, or using that love of the idea of global health to appeal to adcoms.
I apologize if I come off as skeptical, but I've just seen so many people use "interest in global health" as a line on their resume to advance their own careers rather than to really contribute anything meaningful in the field. I hope you gain that global experience, fall in love, and come get your hands dirty with us so we can accomplish some awesome things! Best of luck.