How difficult is to get a 72D Direct Commission?

SSG (Join to see) check the message in RP. Got you an experienced 72D to guide you

SSG (Join to see) you need to work with a medical recruiter, not the regular recruiter. They are the current experts

There are a number of factors, that when combined, will make you competitive for the 72D selection board. I was recently selected during the October 2018 board. You'll need to compile your package via an Army Medical Services recruiter nearest to you (not the guys in the mall). To offer some competition background: they get about 150-200 applicants, per year, for around 10-15 available slots. The vast majority of applicants are people fresh out of college, with a qualifying degree, but no experience. To be very competitive, you'll want some combination of the factors/experience below:

Academics: Having a BS/BA (or higher) in an environmental discipline is the cornerstone to being competitive. Personally, I earned a BS in Environmental Health through Missouri Southern State University (mostly distance learning). A key thing to note (this comes directly from the 72D Consultant): 45 semester hours must be a combination of science and math courses (any combination). Course titles like geology, botany, biology, epidemiology, environmental science, etc., all count towards adding up to 45. GPA is factored in by the board: having a 3.5 or higher would set you apart.
Certification/Licenses: Personally, I had already earned a state-level license as a Registered Environmental Health Specialist, and had the Dept. of Defense Pest Management Applicators License (ties into disease vector control). Any kind of public health, environmental, or related professional certification increases your competitiveness.
Letters of Recommendation: You must have at least 3, but you can have up to 5. At least 3 of my letters were from Lt. Col’s or higher, and 2 were from my academic program. Definitely aim for 5 letters from people that matter.
Military Experience or Civilian: In general, being prior service is a huge plus. Likewise, having completed service schools (NCO Academy, etc), are a plus. If you already work in military health, that will give you a slight edge, especially if your MOS is related to 68S or another preventive medicine or veterinary corps MOS. These Army MOS's are competitive: Medic, Water Treatment Specialist, CBRN Specialist, Preventive Medicine Specialist, and the various veterinary corps specializations. Civilian experience in public health, environmental health, wastewater, food safety, vector control, etc, are also desirable. Regarding the military experience, having ANY military experience, even if unrelated to preventive medicine/public health, is certainly liked by the selection board.
Volunteer Experience: For 4 years, I volunteered as a merit badge instructor for the Boy Scouts, teaching the Environmental Science Badge, and the Public Health Badge. Doing this in your free time really communicates to the selection board that you embody the 72D MOS.
To conclude, all of these factors combined is what got me selected, and definitely made competing much easier. Good luck.