1. Infectious diseases: People with hepatitis A, hepatitis B, hepatitis C, syphilis, AIDS and other infectious diseases can't pass the health examination, and the disease will pose infection risks to others during the travel.
2. Respiratory diseases: Severe respiratory diseases, such as severe asthma and chronic obstructive pulmonary disease, will affect the respiratory function of travelers, unable to adapt to the anoxic environment during long-term flights, and increase health risks during travel.
3. Digestive system diseases: Severe digestive system diseases such as gastric ulcer and intestinal inflammation will affect the digestive function of travelers, which will lead to the deterioration of physical condition during travel and increase discomfort and health risks.