Although it’s not inevitable that you will get diarrhoea (commonly known as ‘Montezuma’s revenge’, ‘Delhi belly’ etc) while you are away, it is pretty likely. Digestive upsets are the most common travel-related illnesses, affecting about 30-50% of travellers to tropical destinations, so you may as well be prepared.
Although there are other causes of travellers diarrhoea, your risk of getting ill mainly depends on how likely it is that the food and drink you are consuming is contaminated with disease-causing microorganisms (although roughly one third of cases are due to non-specific causes such as stress, jet lag and changes in eating habits). If you’re roughing it, and eating from small stalls and restaurants where hygiene is more likely to be a problem, you’re more likely to catch something. If you are away on a long trip, there’s a higher chance you will get ill at some stage.