It's important that medical staff know what illnesses to expect, but educating troops in prevention techniques also goes a long way. Think about planning a trip to Mexico. Your doctor would warn you not to drink the water to avoid diarrhea and other water-borne illnesses.
Army doctors educate troops in the same way, alerting them to common illnesses and ways they can avoid getting sick. They also work on general health education issues, like safe sex and how to handle food properly. An STD or food-borne illness can hinder a soldier's ability just as much as an exotic disease.
Education can make a big difference, but there are some diseases that soldiers can't prevent with techniques like hand-washing and proper food preparation. When doctors can't vaccinate, they sometimes rely on preventive drug therapy instead. More on that on the next page.