Dengue Does Not Bite at Night, Which Is the Whole Problem
The mosquito that spreads dengue is a daytime biter, so the repellent you put on at sunset for the mosquitoes that annoy you is the wrong one.
You get a mosquito bite on a Tuesday. It itches for a day and you forget about it. On Saturday you feel slightly warm and you assume it is the beer. On Sunday you are properly hot and you take paracetamol. By Monday you have a headache behind the eyes and a rash. You go to a clinic and the doctor asks how long you have been in the country, and you suddenly think about that mosquito bite in a way you did not before.
That is roughly the shape of it.
Last verified 5 September 2026.
The bit that catches everyone out
The Aedes mosquitoes that carry dengue bite during the day. They are most active in early morning and the hour or two before dusk, and they are very good at finding ankles, wrists and the back of the knees.
They are not the mosquitoes that wake you at 2am. Those are anopheles, and they carry malaria, which Thailand is essentially free of in the tourist belt. So the single most common piece of mosquito advice given to travellers, put on the repellent at night, is aimed at the mosquito that is not the one you are worried about.
The reason this is not obvious is that the itch is identical. You cannot tell what bit you. So you are treating the itch and the fever as unrelated events, and they are the same event.
What the illness actually does
It is a mosquito-borne viral infection and it is very common in Thailand through the wet season, with the highest transmission in the months around and after the rains.
The first three to four days are a fever that could be anything: sudden high temperature, severe headache, pain behind the eyes, muscle and joint pain that people describe as breaking, nausea, and a rash. It is genuinely indistinguishable from a bad flu on day two and this is why it goes unrecognised.
The fever typically settles after three to seven days, and that is when the dangerous part starts. A rapid improvement can be the crisis period, not the recovery. The critical phase is around defervescence, when the virus leaves and the fluid in your bloodstream drops, and what matters is a blood test showing your platelets.
Get a platelet count if you have had dengue-type symptoms for a few days. The platelet number is the useful number, not the white cell count, and it is cheap and quick in any Thai clinic.
The warning signs that mean hospital, not tomorrow
Severe abdominal pain or tenderness. Not a stomach ache. A notable pain in the upper abdomen that hurts to press.
Persistent vomiting. Repeated, not once, and not a symptom that has eased.
Bleeding from anywhere: gums, nose, under the skin, and bruising that appears without you bumping into anything.
Restlessness, confusion, or cold clammy limbs with a fast weak pulse. This is shock and it is the one to watch for.
Petechiae, a rash that does not blanch when you press a glass against it. Small red spots that stay red.
Difficulty breathing, or chest pain.
Do not treat any of these with paracetamol and see how it is in the morning. Dengue shock can develop in a matter of hours and it is far more dangerous than the fever that announced it. The treatment is fluids and monitoring, and it needs to happen in a hospital, and the reason to know the warning signs is so that you insist on being seen rather than waiting to see whether it settles.
Avoid ibuprofen, aspirin and other NSAIDs entirely if dengue is suspected. They interfere with platelet function and can make bleeding worse. Paracetamol is the analgesic, and aspirin is doubly wrong here because of the dengue risk and because the age of most travellers makes it a poor idea anyway.
Preventing it, which is a small number of hours of work
Repellent with DEET or picaridin, applied in daylight. The most common failure is putting it on once at 9pm and expecting it to have been there all day. The insect bites at 7am.
Long sleeves and trousers, lightly coloured. Aedes are weak fliers and they land on you rather than hovering, and fabric defeats them cheaply.
Shoes. This is the one people skip. The ankle is the most bitten place on a person in Thailand and a pair of socks is more effective than most of the effort spent on the arms.
Check your room for standing water. Water in a bottle, in a bucket, in the ice tray, under the air conditioner, in a flower vase. Aedes breeds in small domestic containers, which is why hotel rooms and Bangkok apartments rather than forests are where you get bitten.
Use the repellent even in daylight if you are somewhere with a known outbreak, and particularly if you are pregnant, elderly, or have any chronic condition, because the risk of severe dengue in those groups is much higher.
What about the vaccine
There is a dengue vaccine used in Thailand, and it has been rolled out in parts of the country, but the licensing history is complicated and the product is not a general traveller vaccine for most visitors. Dengvaxia is licensed in some countries for children with a documented history of dengue infection, because in children who have never had dengue it can increase the risk of severe disease on a first natural infection.
So do not go looking for a jab as a way to make the trip safe. The plan above is what actually works, and it is the same plan that Thai households use. If you are in a higher-risk group, ask your own doctor before you travel rather than asking a clinic in Bangkok.
The mosquito is a nuisance for ten minutes and the illness is a fortnight of misery with a small tail of something much worse, and the ratio of effort to benefit here is about as good as it gets anywhere in travel medicine. Five minutes of daylight repellent and a pair of socks. That is the whole intervention.
