Every country has its own culture, customs -- and germs. Traveling to a foreign country can expose you to diseases like malaria and African sleeping sickness, as well as bacteria your immune system has never been exposed to at home.
Before you travel, familiarize with the illnesses that are common in your destination. The Centers for Disease Control and Prevention (CDC) keeps an updated list of health information for countries around the world, and the World Health Organization (WHO) keeps track of international disease outbreaks. Also take note of conditions in the country you're visiting, such as high altitudes or humidity, which might affect your health.
Check with the CDC or ask your doctor to find out if you need any vaccine(s) beforehand. Try to get vaccinated at least one month before you leave, because some vaccines take a few weeks to reach full potency. Your doctor should give you a checkup to ensure you're healthy enough to travel.
Find out whether your health insurance will cover you if you do get sick abroad. Generally, U.S. health insurance isn't accepted in other countries. It might be worth buying a traveler's health insurance policy specifically for your trip. If you're traveling alone, most policies will cost you less than $100.
To avoid getting sick while you're away, wash your hands regularly with warm water and soap or use an alcohol-based sanitizer gel.
Check with the CDC before you go to find out whether you can drink the water in the country you're visiting. If not, stick with bottled water (be careful about ice cubes, which can also get you sick). Use insect repellant containing the ingredient DEET if you're visiting a country where malaria is prevalent.
If you do get sick and need to find a hospital or doctor, visit the U.S. State Department's list of U.S. embassies and consulates around the world to find a medical professional near where you're staying.