As someone that has degrees in history and library science along with my medical school degree, I would agree with Ian that you can do whatever you want. The main difference between medicine and nursing is that nursing is typically done as a bachelors degree major, but you could always double major in nursing along with another field. There are bachelors to bachelors programs that exist for nursing also, which are typically 12-18 month programs designed for students that have a bachelors degree and want to pursue a bachelors in nursing.
Medical school is different because your bachelors degree is a pre-requisite for medical school, so as long as you take whatever specific courses the medical schools require and take the MCAT you can major in any field. When I interviewed for medical school I heard "We have never had a library science person apply before!" in every interview, so there is definitely something to having a unique background. However, most medical schools get lots of applicants and due to that fact they often select interviewees based on their MCAT score and GPA, but other things are considered. It is very important to earn high marks in your medical school pre-requisites and the MCAT.
Another important thing to do is volunteer, job shadow, or find a job in the health care field. This not only will help your applications, but it will expose you to the day to day work of nurses and doctors so you can verify that this is really what you want to do every day.
Thank you very much for your question, and do not hesitate to contact me if you want clarification on anything I have mentioned.