It's likely all tracks for epi. The JHU PhD admissions process for epidemiology is basically as follows:
1) the admissions committee (admin and faculty) review all of the applications and form a short list.
2) This short list is then sent to the specific tracks that the applicants applied to, and the faculty in those tracks decide from that list who they would like to invite to interview.
3) After interviews, they send their top choice candidates back to the admissions committee.
4) The admissions committee then ultimately decides who gets accepted/waitlisted/rejected.
So the admissions committee probably wants each track to send them their top picks by the end of this month so that they can make their final decisions and send out acceptances in mid-February (in recent years acceptances have gone out ~Feb 13th).
I'm not sure whether this information will reduce or increase stress but I think generally knowledge is power, and I appreciated being told how the process worked by a faculty member when I was applying. I also think knowing the process makes it a little easier to realize that even if you don't get in, it is not personal. There are hundreds of qualified and exceptional applicants and only ~14 spots. It's rough, but please hang in there! I wish this whole forum all the best of luck this admissions season!