How does choosing a specialty work at Medical School?

well you don't choose anything in med school,
1.is general classes then clinical biochemistry,anatomy,physiology,histology,embryology,behavioral sciences, microbiology, pathology,parasitology, and more while you are in medical school you are getting ready to get a degree in general medicine, after you pass your first board depend on the score you will be able to apply to many hospitals or residency programs if they have the program that you want to be in, great !! but all depends on your score. www.usmle.com you'll find more answers. there is 3 exams but the 1 step 1 is very very important, while you doing your residency usually one is doing the specialty in your example infectious disease you will be training your self for few years then you ll have to pass an infectious disease board so you can be certified as an specialist.
seek more info. and Becoming a doctor requires more training than most other jobs. It usually takes at least 11 years to become a doctor: 4 years of college, 4 years of medical school, and 3 years working in a hospital. For some specialties, doctors may have to work in a hospital for up to 8 years before they are trained.
To become a doctor, you should study biology, chemistry, physics, math, and English. It is not easy to get into medical school. You have to do very well in college and on medical school entrance tests.
Students spend most of the first 2 years of medical school in labs and classrooms. They take lots of science courses. They also learn to ask patients the right questions and how to examine them. They learn how to tell what sickness a patient has. In the last 2 years, students work with patients and doctors in hospitals and clinics. After medical school, doctors go to work in a hospital for a few years. They are called residents. To be a resident, you must take a test.
If you want to be a doctor, you should like to help people. You should be willing to study a lot.
You have to know how to talk to sick people. And you should be able to make decisions and handle emergencies

GOOD LUCK

You don't usually specialize until after you graduate medical school.

If I were you, I would get into medical school, then pay close attention in pathology, microbiology, and medicine classes.  When on rotation, make it a point to search out infectious disease (ID) specialists and learn their trade.

I am not certain, but I believe that most infectious disease specialists begin an internal medicine residency and then move on to infectious disease.  I am unsure if they are required to finish the internal medicine residency and I am unsure if infectious disease is its own residency or a fellowship (or both or either.)

There's a matching program for residencies. You'll get a chance to go through a lot of clinical specialty rotations before you get there, so you'll have some idea, then you set up interviews and submit yourself to the match, For ID, you'll do an internal medicine residency and then an ID fellowship.

Medical school is generally divided into two parts: the first 2 years are spent learning the medical sciences (physiology, pathology, anatomy, physical examination, etc...); the second 2 years are spent doing clinical rotations through various disciplines of practice (pediatrics, internal medicine, surgery, psychiatry, etc...).  At the end of your third year you will have to begin making the decision about what specialty you would like to pursue.  During your fourth year you will go through a process known as "The Match."  The Match is a nationwide system of "matching" applicants to specific Residency training programs across the country.

  Residency is where you truly graduate from being a "doctor" to being a "physician."  It is a truly life-changing experience and will either make or break you in your career.  Upon completion of your residency you have two options: practice medicine in your current specialty or enter a Fellowship.  A Fellowship is a further step in medical training in a very specific Subspecialty of medicine. (This is where Infectious Disease comes in.)  Infectious Disease Fellowships are most commonly a branch of Internal Medicine, but there are also Pediatric Infectious Disease Fellowships as well.

  So, my advice...you've got a long way to go.  Take it one step at a time and keep an open mind.  I entered medical school expecting to do Cardiology or Pulmonology... I'm now a Pediatrician and wouldn't want to do anything else.
   Best of luck!

many medical students have no idea what they want to do when they graduate. they just have a vague idea that they want to be some kind of doctor and they try to pick a field that interests them during the course of their training. others are very focused, and they know exactly what field they want to specialize in before even entering medical school- for example maybe your father was a family doc and you want to take over his business. neither approach is wrong. if you already know that you want to specialize in i.d., most medical schools have elective rotations during your 3rd or 4th year. you should be able to choose an i.d. rotation at that time. after graduation you would need to do an internal medicine residency followed by an i.d. fellowship.