JHU doesn't have a pre-med major or set curriculum; rather, you fulfill your pre-med "requirements" by choosing courses that would count toward whatever medical school you'd like to apply to (for instance, a year of math, two years of bio, etc. etc.). Pre-meds often choose Bio or Public Health majors because there is a lot of overlap in major requirements with med school requirements, and also the classes tend to prepare you for the MCATs -- but I would make sure that you hold an interest in whatever major you decide.
And you definitely don't have to commit right off the bat! If you change your mind later, there's always the possibility to switch majors too.
Signed,
Former Bio pre-med who switched into Psych after freshman year and is still graduating a year early