When driving, be sure to wear seat belts, When flying, keep your seat belt on to avoid injury in the event of sudden turbulence. Serious illness during travel is most commonly due to pre-existing conditions such as coronary heart disease, hypertension, diabetes or asthma. The medicines you normally take may not be readily available during your travel, so bring plenty for the entire trip. Altitude sickness is common at altitudes greater than 14,000 feet and may occur at at altitudes greater than 8,000 feet. Symptoms include headache, poor appetite, sleep disturbance and just not feeling well.
Shortness of breath is an ominous sign.
Mild symptoms generally resolve spontaneously in a few days. More severe illness should be treated by descent to a lower altitude. Generally, you need one day for each time zone crossed to adjust to the new time zone. If you begin your trip overtired, stressed or hungover, it will take longer to adjust. Melatonin appears to be somewhat effective in treating jet lag, however I don't recommend it because the amount of melatonin in different preparations is not standardized, making it impossible to know whether the preparation you buy will be effective or not.