When troops are going to be deployed, the Army doctors that accompany them do research on the location and the enemies they'll be facing to see what sorts of illness soldiers will be encountering in the field.
Many infectious diseases are location specific, so doctors prepare for preventing and treating those illnesses before the troops are deployed. They rely on information from agencies like the Centers for Disease Control and Prevention to get information about what to expect. Depending on where the troops are headed, how long they'll be there and what they'll be doing, Army doctors assess the risk of infectious disease and try to ensure that they have the medical staff and supplies on hand to address any issues that pop up.
Of course, educating the troops is also important! More on that next.