Malaria:
Malaria is the number one life-threatening infectious diesase of international travelers according to the CDC. You need to see your healthcare provider 4-6 weeks before travel to an area where Malaria is a problem.

Risk according to WebMD:

• The risk is highest in sub-Saharan Africa, Papua New Guinea, the Solomon Islands, and Vanuatu.
• The risk is intermediate in Haiti and the Indian subcontinent.
• The risk is low in most of Southeast Asia and Latin America.

See:
Air Travel Health - Jet Lag
Traveler's diarrhea (TD)
Acute Mountain Sickness (AMS)
Motion Sickness
Tips for Healthy Travel at Continental
Causes of Death