General Travel Precautions
On the whole, travel to East Asia and particularly to cities in the developed areas poses no great health dangers. According to the Information for Travelers to East Asia put out by the National Center for Infectious Diseases Health: “The preventive measures you need to take while traveling in East Asia depend on the areas you visit and the length of time you stay.... However, in highly developed areas of Japan, Hong Kong, South Korea, and Taiwan, you should observe health precautions similar to those that would apply while traveling in the United States.”
The situation in China is different, but generally speaking there are no great health hazards in the developed cities other than poor water and air quality. For more specific information on vaccinations and disease prevention, see the guidelines for travel in East Asia put out by the United States Center for Disease Control online at http://www.cdc.gov/travel/eastasia.htm.
Food and Water Precautions
A similar distinction obtains with regard to water and food. In Japan and other developed parts of East Asia, food and drink are generally as safe or safer than in the United States. But caution must still be exercised in China, even in the larger cities. Food and drinks prepared in expensive Western style hotels are usually safe. But elsewhere it is wise to follow some basic precautions to avoid diarrhea and more serious problems. The Center for Disease Control recommends the following measures:
Water
"In areas with poor sanitation, only the following beverages may be safe to drink: boiled water, hot beverages (such as coffee or tea) made with boiled water, canned or bottled carbonated beverages, beer, and wine. Ice may be made from unsafe water and should be avoided. It is safer to drink from a can or bottle of beverage than to drink from a container that was not known to be clean and dry. However, water on the surface of a beverage can or bottle may also be contaminated. Therefore, the area of a can or bottle that will touch the mouth should be wiped clean and dry. In areas where water is contaminated, travelers should not brush their teeth with tap water."
Food
"Food should be selected with care. Any raw food could be contaminated, particularly in areas of poor sanitation. Foods of particular concern include salads, uncooked vegetables and fruit, unpasteurized milk and milk products, raw meat, and shellfish. If you peel fruit yourself, it is generally safe. Food that has been cooked and is still hot is generally safe.... Some fish are not guaranteed to be safe even when cooked because of the presence of toxins in their flesh."
Further Sites to Consult
U. S. Consular Information Sheet on China, Sections on Safety and Health
U. S. Consular Information Sheet on Japan, Sections on Safety and Health
Health Information for International Travel, the CDC "Yellow book" gives immunization requirements, disease risks, and prevention information for travel to other countries.