Washing your hands thoroughly, drinking bottled water, eating carefully, applying sunscreen… are things which will help you healthy avoid being sick while traveling.
Wash your hands thoroughly

Before eating or after using the toilet, you need to wash your hands to minimize the risk of infection. Tourists should bring their own bottles of hand sanitizer so they can use them in whichever situations.
Drink bottled water

When coming to the areas where you are not sure whether water for drinking is hygienic enough, it is better for you to use bottled water to avoid the spread of many diseases. The fact that local people can drink the water at an area doesn’t mean that it is safe for you to do that because your stomach may not have the appropriate bacteria. Moreover, you should not drink unpasteurized milk. Many types of tea, coffee and beer are pretty safe.
Beware of food poisoning

Food poisoning can make your holiday become a disaster. Therefore, you should be careful when eating, avoid eating fresh vegetables which don’t have skin and salad with unhygienic water and all types of raw food (especially meat and seafood).
Avoid being bitten by mosquitoes

Aside from making you feel uncomfortable, the mosquito bites can be extremely dangerous. Mosquitoes are the vector of many infectious diseases such as yellow fever, malaria, Zika. Therefore, you should use sprays or lotions that repel mosquitoes, sleep with a fan or mosquito net, wear long-sleeved clothing to reduce the risk of being bitten by mosquitoes.
Seek medical advice

Nobody knows more about your health than you. If you feel that you need assistance from a doctor or medical professional before you go traveling, you should do it. Some tourist destinations have bigger risk involved with health than others. The doctors can give you advices on the necessary vaccines.
Protect your skin from the sun

You should use sunscreen with SPF of 15 or higher. You should also wear hat, provide enough water for your body after exposed to sunlight for a long time, to avoid sunburn or sunstroke.