What Is Diarrhea? Causes, Types, and When to See a Doctor

Clinically reviewed by Dr. Ponlawat Pitsuwan, Physician, Doctor Samui. Last reviewed: August 2026

You were fine at breakfast. By mid-afternoon, after a plate of food from the night market near Chaweng Beach, you are not leaving the bathroom. This is the situation we see multiple times a week at our clinic. Diarrhea is the most common illness we treat in tourists on Koh Samui, and most cases are straightforward. But some are not, and knowing which is which matters when you are far from home.

Diarrhea is simply your gut moving things through faster than it should, so your body cannot absorb water properly. The result is loose, watery stools, sometimes with cramps, nausea, or a low fever. The cause determines how serious it is and whether you need medication or just time and fluids.

What Causes Diarrhea?

On Koh Samui, food and water are the usual starting point. Bacteria like E. coli and Salmonella thrive in tropical heat. Food that sits out for an hour in 33°C humidity can become a problem fast. Vibrio bacteria, found in raw or undercooked seafood, are common here and can hit harder than a typical stomach bug.

Viruses are the other big cause. Norovirus spreads easily between people in close spaces like resorts, dive boats, and tour buses. Rotavirus is the main culprit in young children. Both cause sudden watery diarrhea, often with vomiting.

Parasites take longer to appear. Giardia and Cryptosporidium can come from contaminated water. They tend to cause diarrhea that comes and goes over weeks rather than a sharp hit in the first 48 hours. If you have been unwell for more than a week, a parasite is worth testing for.

Some cases have nothing to do with infection. Antibiotics disrupt gut bacteria and commonly cause loose stools. Anxiety and jet lag can trigger bowel changes too, and some people react to new spice levels they are not used to.

Types of Diarrhea Explained

The simplest way to think about diarrhea is by how long it lasts. Acute diarrhea lasts fewer than fourteen days and covers most holiday stomach bugs. Persistent diarrhea runs from fourteen to thirty days. Chronic diarrhea means it has been going on for more than thirty days and needs proper investigation.

There are also different types based on what is going wrong in the gut. Osmotic diarrhea happens when something pulls water into the gut that should not be there, like certain medications or unabsorbed sugars. Secretory diarrhea means the gut is pushing water out even when you are fasting. Inflammatory diarrhea involves damage to the gut lining, often from infection, and usually produces bloody stools.

For most tourists on Koh Samui, what they have is acute infectious diarrhea, bacterial or viral, that resolves within a few days. Knowing the type matters because it points toward different causes and different treatments.

What Is Traveler’s Diarrhea and Why Is It So Common on Koh Samui?

Traveler’s diarrhea is what happens when your gut meets bacteria or parasites it has never encountered before. Your immune system has no memory of the local strains of E. coli or Salmonella that locals have built tolerance to over years. So you get sick, and they do not.

Koh Samui has specific risk factors that make this worse. The heat accelerates bacterial growth in food. Ice is often made from tap water, which is not safe to drink here. Reef water and lagoon water around Angthong Marine Park can carry bacteria that cause stomach illness if swallowed while snorkeling. We see this pattern constantly from May through October when tourist numbers peak.

Most traveler’s diarrhea starts within the first three to five days of arrival. It usually brings cramping, urgent loose stools, and sometimes a low-grade fever. The danger is not the diarrhea itself. It is dehydration.

Diarrhea and Dehydration in a Tropical Climate

This is where Koh Samui changes the equation. In a cooler climate, diarrhea alone is the main fluid loss. Here, you are already sweating out fluid all day in thirty-plus degree heat and high humidity. Add diarrhea on top, and you can become seriously dehydrated within hours.

Watch for dark urine or no urination for eight or more hours, dizziness when standing, dry mouth, and feeling confused or very weak. If those signs appear, oral rehydration alone may not be enough. An IV drip becomes the faster, more effective option. Doctor Samui offers IV fluid therapy at our Chaweng clinic and through hotel and villa visits across the island.

Oral rehydration salts are your first line at home and are available at most pharmacies here. Coconut water is refreshing but is not a medical substitute for rehydration salts. Plain water alone does not replace the electrolytes you are losing.

Diarrhea in Children: What Parents Travelling with Kids Need to Know

Children dehydrate much faster than adults. A toddler can go from mildly unwell to seriously dehydrated in a few hours, especially in Koh Samui’s heat. This is not a situation to watch and wait on.

Rotavirus is the most common cause of severe diarrhea in children under five. It hits fast with watery stools and vomiting together. The main danger is fluid loss, not the virus itself.

Call a doctor right away if your child has not had a wet nappy in three or more hours, has sunken eyes or a dry mouth, has a fever above 38°C if under three months or above 39°C if older, or is unusually sleepy and hard to wake. Doctor Samui provides paediatric care and can come directly to your hotel or villa so you do not have to carry a sick child across the island.

When Should You See a Doctor for Diarrhea on Koh Samui?

Most healthy adults can manage mild diarrhea at home for the first twenty-four to forty-eight hours. Drink oral rehydration salts, rest, and avoid dairy, caffeine, and alcohol. Plain foods like bananas, rice, and toast are easy on the gut and a good starting point.

See a doctor if your diarrhea has not improved after two days, if you see blood or mucus in your stools, if your fever climbs above 38.5°C, or if you cannot keep any fluids down. Also come in if you have a pre-existing condition like diabetes or inflammatory bowel disease, where diarrhea can escalate faster.

Loperamide, sold as Imodium, can slow diarrhea and is fine for short-term use in adults when you need to travel or sleep. Do not use it if you have a fever or bloody stools. It is not right for children without medical advice.

Probiotics are safe and some people feel they help recovery, but the evidence for shortening infectious diarrhea is limited. They are worth taking, but they are not a treatment on their own.

Getting Diarrhea Treatment on Koh Samui

If you need a doctor, you have two straightforward options. Walk into our clinic in Chaweng, where we can run a stool test if needed, check your hydration, and start treatment including antibiotics if the cause is bacterial. Or message us on WhatsApp and we will come to you, anywhere from Maenam and Bophut in the north to Lamai and Hua Thanon in the south.

A stool test takes one to two days for full results, but we can often make a clinical diagnosis and start treatment the same day based on your symptoms. If you are dehydrated, we can set up an IV drip on-site. You do not need to go to hospital for most cases of diarrhea here.

For more detail on what we check and how we treat it, see our full guide to diarrhea treatment in Koh Samui.

What is the difference between diarrhea and traveler’s diarrhea?

Diarrhea is a general term for loose or watery stools three or more times a day. Traveler’s diarrhea is a specific type caused by bacteria, viruses, or parasites your gut has not encountered before. On Koh Samui, the most common triggers are E. coli, Salmonella, and Vibrio from local food and water. The treatment approach is similar, but identifying the type helps us decide whether antibiotics are needed.

How do I know if my diarrhea needs a doctor or if I can manage at home?

If you have had diarrhea for more than two days without improvement, see blood in your stool, have a fever above 38.5°C, or cannot keep fluids down, you need a doctor. Dizziness, no urination for eight or more hours, or feeling confused are signs of serious dehydration. On Koh Samui, there is no reason to push through it alone when a doctor can come to your hotel.

What should I eat and drink if I have diarrhea on Koh Samui?

Start with oral rehydration salts, available at most pharmacies here. Plain foods like bananas, rice, and toast are easy on your gut. Avoid caffeine, alcohol, dairy, and spicy food until you are fully recovered. Coconut water is not a substitute for rehydration salts when you are losing fluid fast through both sweat and diarrhea.

Can diarrhea be serious in children? What should I do if my child has diarrhea on Koh Samui?

Yes, diarrhea is more dangerous in young children because they dehydrate much faster. Watch for no wet nappy in three or more hours, sunken eyes, dry mouth, high fever, or unusual drowsiness. Doctor Samui offers paediatric care with hotel and villa visits so you do not need to transport a sick child across the island.

Is diarrhea after snorkeling or swimming in the sea normal?

It is not unusual. Swallowing seawater while snorkeling or diving around Koh Samui’s reefs can introduce marine bacteria, including Vibrio species, that cause rapid stomach symptoms. Symptoms typically appear within hours to a day. If diarrhea continues beyond twenty-four hours or you develop a fever, come in for assessment, as marine bacterial infections can move quickly.

Is loperamide (Imodium) safe to take for diarrhea in Thailand?

Loperamide is safe for short-term use in healthy adults when you need to travel or sleep. Do not use it if you have a fever or blood in your stools, as it can slow the clearance of the infection. It is not right for children without a doctor’s guidance. If you are unsure whether your diarrhea is bacterial, a quick assessment before self-medicating is the safer call.