Traveling can increase your chances of getting sick. A long flight can increase your risk for deep vein thrombosis. Once you arrive, it takes time to adjust to the water, food, and air in another place. Water in developing countries can contain viruses, bacteria, and parasites that cause stomach upset and diarrhea. Be safe by using only bottled or purified water for drinking, making ice cubes, and brushing your teeth. If you use tap water, boil it or use iodine tablets. Food poisoning can also be a risk. Eat only food that is fully cooked and served hot. Avoid unwashed or unpeeled raw fruits and vegetables.
If you are traveling out of the country, you might also need vaccinations or medicines to prevent specific illnesses. Which ones you need will depend on what part of the world you're visiting, the time of year, your age, overall health status, and previous immunizations. See your doctor 4 to 6 weeks before your trip. Most vaccines take time to become effective.
Centers for Disease Control and Prevention
References and abstracts from MEDLINE/PubMed (National Library of Medicine)