Koh Chang, Koh Mak and Koh Kood sit in a province where the CDC lists malaria medicine.

Yes. Koh Chang, Koh Mak and Koh Kood have the day-biting Aedes mosquito that spreads dengue, and sandflies bite on some quiet beaches at dawn and dusk. All three islands belong to Trat, a province on the Cambodian border, and the CDC lists malaria prevention medicine for the provinces along that border, so this beach trip starts with a travel clinic. Our Thailand briefing covers the rest of the country.

Malaria in a border province, in the CDC's words

The CDC's Thailand page, reviewed in October 2026, places malaria transmission "Primarily the provinces that border Burma (Myanmar), Cambodia (rare cases in Buri Ram Province), and Malaysia (especially in the rural forest and forest-fringe areas of these provinces)." For prevention medicine it lists "Provinces that border Burma (Myanmar), Cambodia (except Buri Ram Province), and Malaysia: Atovaquone-proguanil, doxycycline, tafenoquine." Trat borders Cambodia, and all three islands belong to it.

The malaria mosquito bites from dusk to dawn, so wear repellent and long sleeves after sunset and sleep behind screens or under a net, which matters more here than on Koh Samui or Phuket, where the CDC recommends no malaria medicine. Our malaria-zone checklist covers the rest.

Dengue, and Trat's bad year

Dengue is endemic, and Trat can flare. Department of Disease Control surveillance figures put Trat's dengue rate at 502.5 cases per 100,000 people by early August 2023, the highest of any Thai province. Trat was not among the highest-rate provinces in 2024 or in the department's July 2026 ranking. Nationally the department counted 25,026 cases by September 15, 2026, about 1.8 times fewer than in 2025, though cases were still rising in the south.

The dengue mosquito bites in daylight around villages, resort gardens and bungalows, so put repellent on over sunscreen in the morning, after swims and before the late-afternoon peak. Our dengue & Zika routine has the schedule. The same mosquito carries chikungunya, which the CDC rates an elevated risk in Thailand.

Sandflies, island by island

Our Thailand briefing treats the tiny beach biters that Thai islands call sandflies as biting midges, a nuisance rather than a disease risk. They tend to be worst on Koh Mak's west-coast beaches, milder on Koh Kood and scarce along Koh Chang's main resort strip, though they can turn up on any of the three, especially on sheltered, unraked sand toward dusk. Treat ankles and calves before you sit down, pick a raked, breezy stretch in front of a resort and use a lounger or mat. Leave bites alone, and see a doctor if one turns red, hot or swollen. Our Barbados sandfly guide explains how they behave.

When to go

The dry, calm season runs roughly November to April and is the easiest stretch for bites and boat crossings. From about May to October the southwest monsoon brings heavy rain, some Koh Mak and Koh Kood speedboats and resorts cut back, and Thailand's dengue usually peaks.

Where you're going

What travelers report

Paraphrased from TripAdvisor forum threads and linked so you can read them in full. None is dated, and sandflies come and go with the beach, the wind and the season.

• Koh Samui and Phuket main beaches (Ko Mak thread)

  Comparing islands, a poster said sandflies gather where the sand is not raked and few people walk, that the main beaches on Samui and Phuket are built up with heavy foot traffic, and that lying in front of a hotel rather than at the deserted end of a beach avoids them. TripAdvisor ↗

• Koh Mak, Koh Kood and Koh Chang

  Asked whether Koh Mak and Koh Kood still had a sandfly problem, a longtime forum contributor said it continued on Koh Mak's west-coast beaches while the east coast was clear, that Koh Kood's problem was milder and Koh Chang had almost none, and that they had been bitten on Klong Chao beach. TripAdvisor ↗

• Koh Kood

  In a thread asking whether there were still sandflies on Koh Kood, a forum expert said they are present but that it depends on the time of day, the wind and how much the sand is disturbed, and another poster had not noticed any on recent trips. Suggestions included a strong DEET product with sunscreen and antihistamine or hydrocortisone cream for bites. TripAdvisor ↗

One rule outlasts the trip. Any fever during or after your trip means a doctor visit, and you tell them you were in Trat, on the Cambodian border. Thailand's Department of Disease Control advises paracetamol (acetaminophen) rather than ibuprofen or aspirin until dengue is ruled out.

Traveler questions