Some vaccines must be given in a series over several days or weeks, the CDC says. The vaccine for yellow fever may take up to 12 days to develop antibodies.

Hepatitis A can be contracted anywhere, including the United States. Infection is less likely in Western Europe than in less developed countries. Vaccination is not recommended for travel to Western Europe.

If you can, walk up and down the aisle of the plane. If traveling by bus or train, make the same effort. If traveling by car, stop and walk around every hour or so, advises the National Heart, Lung, and Blood Institute. Movement prevents blood from pooling.

The best choice is the U.S. Embassy. Embassy staff can recommend English-speaking doctors or the best hospital in your locale.

Choose water over the other beverages listed. Caffeinated and alcoholic beverages dehydrate the body by speeding up the rate of urination. For optimum rehydration, drink one cup of water for each hour of flight time.

If you are in an area where water may be contaminated, don't use ice in your beverage, because the ice may be contaminated. It's safest to drink directly from the bottle or can, rather than from a glass, the CDC says. Also be careful of water on the outside of the bottle or can. Wipe the bottle or can dry before opening, and clean the area where your mouth will touch.

Avoid uncooked vegetables and moist dishes such as rice that could have been sitting at room temperature.

Mixing alcohol with a soft drink doesn't kill any organisms in the latter. And the ice may be made from tap water.

Keep any medications in their original bottles and carry a copy of your prescriptions, along with a note from your doctor if you take controlled substances.