The oysters were three dollars and the man shucking them was fast and cheerful and I watched him do it, which I told myself counted for something. Fourteen hours later I was on the bathroom floor of a guesthouse in Hoi An doing arithmetic about how far the bus station was. I had been careful all week. That is the part nobody warns you about — being careful is not the same as being safe.
It happens to careful people, and that is not me being kind
Somewhere between 30% and 70% of travellers get diarrhoea during a two-week trip, depending on where they go and what season it is. It is the single most predictable travel illness there is. Not bad luck. Not a weak stomach. Just arithmetic.
You have probably heard the rule — boil it, cook it, peel it, or forget it. It is not useless, but studies have found that people who follow it still get sick. The biggest driver is not the ice in your drink. It is hygiene in the kitchen: how hands get washed, whether the fridge held temperature through the afternoon blackout, what the cutting board touched before your food. None of that is visible from your side of the counter, and no amount of careful ordering fixes it.
I find this genuinely useful to know, because the alternative is spending your trip suspicious of every meal and still getting ill anyway. Eat sensibly. Then stop auditing yourself.
How long after the meal? That narrows it down a lot
The gap between eating and feeling terrible tells you roughly what you are dealing with. It is the most useful thing you can work out on your own.
| Time from meal to symptoms | Usually means | How long it runs untreated |
|---|---|---|
| A few hours | A toxin already in the food | Short and violent, then over |
| 6 to 96 hours | Bacteria or a virus | Bacteria 3–7 days · virus 2–3 days |
| 1 to 2 weeks | A parasite | Weeks to months if untreated |
Bacteria cause most of it — somewhere around 75% to 90% of cases. Viruses account for another 10% to 25%, and they tend to come with more vomiting. Parasites are the slow ones, roughly one case in ten, and they mostly show up in people who have been travelling for a while rather than someone on a two-week holiday.
So: sick the same evening as a specific meal, and it was probably that meal. Sick three days in, and you can stop blaming the oysters and stop interrogating your travel partner about where they ate lunch.
Stop counting trips to the toilet
Most advice online tells you to count how many times you go. Current guidance dropped that approach years ago, and replaced it with a better question: is this stopping you doing things?
| How bad | What it means | What that changes |
|---|---|---|
| Mild | Annoying, but you can still do your day | No antibiotics. Fluids, and loperamide if you want to sit on a bus without fear |
| Moderate | Distressing, or it has wrecked your plans for the day | Antibiotics become a reasonable option — a doctor decides which |
| Severe | You cannot function. Any blood in the stool counts as severe automatically | Antibiotics advised, and you want someone qualified looking at you |
I like this scale because it matches how you actually experience it. Nobody counts. Everybody knows whether they cancelled the day.
One thing worth knowing about loperamide, the anti-diarrhoeal most people carry: it is fine and genuinely helpful for a mild case or a long bus ride. But if you have blood in your stool or a fever, do not take it on its own. Slowing everything down while an invasive infection is in there is the wrong move.
🔴 Call 115 or get to a hospital now if any of these are true: you vomit blood; your vomit looks like ground coffee; your skin, lips or tongue turn blue, grey, pale or blotchy; you are struggling to breathe.
🟠 See a doctor the same day if: you still look dehydrated after using rehydration sachets; you cannot keep any fluid down; there is blood in your stool; the diarrhoea has run past 7 days, or the vomiting past 2 days.
Signs of dehydration to watch for: dizziness, muscle cramps, sunken eyes, very little urine, a dry mouth, weakness, a fast heartbeat.
The rehydration part almost everyone gets wrong
This is the one thing that actually matters in the first day, and it is the thing most people do badly — usually by reaching for whatever is coldest and sweetest in the hotel fridge.
- If you are losing a lot of fluid, use proper oral rehydration salts. One sachet into the volume on the packet, normally one litre of boiled or bottled water. Any pharmacy in Vietnam has them, they cost almost nothing, and yes, they taste like the sea.
- If it is mild, honestly, drink whatever you will actually drink. Water, weak tea, a sports drink. The best rehydration fluid is the one you finish.
- Do not sit there drinking cola or fruit juice. Very sweet drinks pull water into your gut and can make the diarrhoea worse. This is the mistake I see most often.
- Sip small amounts often if you feel sick. Large gulps come straight back up.
- Eat when you feel able to. Skip fatty and spicy food for a day or two. You do not need to fast.
When it is not just a stomach bug
Here is the part that makes Vietnam different from getting ill at home, and the reason I would not just wait it out.
Dengue is a year-round risk across much of the country, and early dengue can look like a rough stomach and a fever. Typhoid spreads through exactly the same food and water route as ordinary traveller's diarrhoea — it is common enough here that vaccination is recommended for most visitors, and the vaccine is not fully protective, so it stays on the list even if you had the jab. Hepatitis A travels the same way.
None of that means your bad night is something serious. It usually is not. It means that diarrhoea plus a fever in Vietnam is not automatically "something I ate", and that is worth ten minutes with a doctor rather than three more days of guessing.
What a doctor actually does about it
For most people, a visit is not about getting a prescription. It is about someone competent deciding which of the three boxes above you are in, and ruling out the things that look similar.
- Examining you properly and checking how dehydrated you actually are — this is difficult to judge about yourself
- A blood test on the spot if a fever needs explaining, which is what separates a stomach bug from something that needs treating differently
- IV fluids if you genuinely cannot keep liquid down. Not as an upgrade — as the thing you do when drinking has stopped working
- An antibiotic only if the picture calls for one, and the right one, rather than whatever a pharmacy hands over the counter
- A written note and an invoice in English, which your travel insurance will want
A home or hotel visit is worth more here than it sounds. Getting into a taxi across a city you do not know, in the state you are in, to sit in a waiting room, is genuinely awful — and it is also how you end up delaying until day five.
What it costs
A doctor coming to your hotel, Airbnb or apartment starts at $57. If you need IV fluids or injections, that starts at $38. You get a price before anyone sets off, and card payment is accepted. Ask for the English invoice while the clinician is still with you — it is much harder to chase afterwards.
The short version
Drink properly, ignore the urge to count, and pay attention to two things: blood, and fever. Most cases settle in five to seven days, with the vomiting stopping well before the rest. If you are not clearly improving by day three, or if a fever turns up, stop waiting it out and get someone to look at you. In Vietnam that is a phone call, not an expedition.
Sources: CDC Yellow Book 2026 — Travelers' Diarrhea; CDC Travel Health — Vietnam destination page; CDC — Food and Water Safety for travellers; NHS — Diarrhoea and vomiting.
