“Don’t drink the water!” – A refrain from anyone who has visited a developing nation that lacks the infrastructure for clean-ish drinking water.
I can speak about this one on a personal level because I forgot about this crucial piece of advice when I visited Vietnam. I made the mistake of ingesting some of the local water and paid for it with a couple of day’s worth of diarrhea as my body sorted out why it was being assaulted. In the same period, I had managed to sustain a rather large gash on my hand that needed to be bandaged and treated for infection proactively. I found my way to a drugstore and I was able to buy prescription-strength antibiotics and ointment over the counter because the restrictions on such things are a lot looser in Vietnam, thankfully.
Let this serve as a precautionary tale. Your body may have a similar reaction to the food and drink of your destination and there is nothing inherently wrong with this. Your body is adjusted to the environment in which it spends the most time, and if you find yourself in a place with different standards of cleanliness, you must remain diligent. Do your research and pack accordingly. Leave room in your kit for any supplies you feel would help you stay happy and healthy.
If the water is even a possible gray area, bring along some treatment in the form of tablets or filters, or consider purchasing bottled water in your destination. When purchasing bottled water, be sure to check the seal for any tampering because it isn’t uncommon to find bottles refilled locally and sold as fresh from the factory.
It’s just one example, but be proactive and you’ll reduce the chances that you spend the trip retching in the bathroom the whole time.