There are two main categories of travel illness: motion sickness and traveller's diarrhoea. The first of these is caused by too many signal being sent to the brain and then not making sense together. This triggers a vomiting response. You can get medication which shuts down this response to calm your motion sickness, which may otherwise make travelling long distances very difficult for you. Motion sickness is most common among children, but most grow out of that. In adults it is more common among women.
When you are travelling abroad you will be exposed to new kinds of bacteria in your food and water. These may even be bacteria that perfectly harmless to the people in the area, but because it is a change what you're used to may cause an upset tummy for you. This can lead to diarrhoea and illness, which while not usually life-threatening, can ruin your holiday especially if it means you need to come home. However there are travel sickness tablets and motion sickness pills which you can buy online to manage these problems. Travel sickness tablets contain antibiotics which fight bacteria in your stomach.