Among the potentially serious diseases that international travelers may be exposed to…

H1N1 (swine) flu. The virus experienced a resurgence in Mexico earlier this year. Flu season has begun in South America and other parts of the Southern Hemisphere. Ordinary flu shots protect against the virus. Booster shots are recommended for people who are traveling there from now through September.

Dengue. Outbreaks of this mosquito-borne viral disease have been reported recently in the Caribbean, Southeast Asia and Central and South America. No vaccines or other preventive treatments are available. To protect yourself when going outdoors, use controlled-release insect repellent containing 20% to 30% DEET. Permethrin repellent can be sprayed on clothing, bedding and mosquito nets.

Malaria. This is another mosquito-borne disease—but one for which prophylactic drugs are available. Among the travelers at risk: People on African safaris…visitors to parts of South America and southern Asia. Antimalarial drugs are not 100% effective, so you still must protect yourself against mosquitoes.

Hepatitis A. This viral disease is transmitted from person to person and through contaminated food or drink. Most travel-related cases are associated with Mexico and Central and South America. Vaccines are available.

Self-defense: Four to six weeks before an international trip is the optimal time to visit a travel clinic or other health-care provider to start any necessary immunizations or other preventive treatments. But if you must leave sooner, immunizations still are recommended.