No malaria pills for Koh Samui or Koh Pha Ngan. Plan around the day-biting dengue mosquito and the Gulf's late rains.

Yes. Koh Samui and Koh Pha Ngan have the day-biting Aedes mosquito that spreads dengue, in every month of the year. The CDC reports only rare to few malaria cases on both islands and recommends no malaria medicine, so the job is a daytime repellent habit, with extra care in the Gulf's October-to-December rains.

Dengue and the Gulf's late rains

We couldn't find a 2026 dengue count for Surat Thani, the province that takes in both islands, and it was not among the five provinces that Thailand's Department of Disease Control named for the highest dengue rates in its July 27, 2026 release. Nationally the department counted 25,026 cases and 39 deaths by September 15, about 1.8 times fewer than in 2025, but said cases were still trending up, especially in the south.

On October 22, 2026, the department's regional office in Nakhon Si Thammarat warned of continuing heavy rain and flash floods across the south, listed dengue among four disease groups to watch, and advised mosquito repellent or sleeping under a net. In the rains, empty any standing water on your balcony and sleep in an air-conditioned or screened room.

The mosquito bites in daylight around resorts, bungalows and beach bars, so repellent goes on over sunscreen in the morning, after swims and before the late-afternoon peak. Our dengue & Zika routine has the full schedule. The same mosquito carries chikungunya, which the CDC rates an elevated risk in Thailand, and Zika, which circulates at low levels.

Malaria: no pills for either island

The CDC's Thailand page reports "rare to few cases" of malaria "on the islands of Koh Pha Ngan, Koh Samui, or Phuket." Surat Thani doesn't border Myanmar, Cambodia or Malaysia, so the CDC's line for "All other areas with malaria transmission" applies: "No chemoprophylaxis recommended (insect bite precautions and mosquito avoidance only)." Our Thailand briefing maps the border provinces where the CDC does list medicine. Yellow fever vaccine is not recommended.

Beaches, bays and Full Moon nights

On Samui, visitors mostly mention mosquitoes. Sandflies, which our Thailand briefing treats as biting midges, gather on quiet beaches where the sand isn't raked, and the island's main beaches are built up and busy. On Koh Pha Ngan, a Bottle Beach reviewer mentioned sand fleas, the only such report from either island, so treat ankles on the quieter northern bays at dawn and dusk.

Full Moon Party nights at Haad Rin run from dusk to dawn, mostly after the dengue mosquito's late-afternoon peak. The bars and bungalows behind the beach tend to have mosquitoes, so put repellent on at dusk.

The stings in the water are a separate problem. The "sea lice" swimmers describe at Haad Rin and Chaweng are generally blamed on tiny jellyfish or anemone larvae, and repellent won't stop them. The same regional office warned in August 2026 that venomous jellyfish are more common in the rainy season, so watch for warning signs and jellyfish nets.

When to go

Samui and Koh Pha Ngan tend to get their heaviest rain from about October to December, with the northeast monsoon, while the Andaman coast is wet from about May to October. That makes roughly January to April the easiest stretch. Much of the middle of the year tends to be drier here than on the Andaman side, though June to September is when Thailand's dengue usually peaks. Dengue is reported in every month.

Where you're going

Adding the Andaman coast? The Phuket & Krabi briefing covers the other side.

What travelers report

Paraphrased from TripAdvisor forum threads and linked so you can read them in full. Neither is dated, and bites on the Gulf islands change with the beach, the wind and the rain.

• Koh Samui

  A poster said they had never seen sandflies on Samui but there are definitely mosquitoes, and that how they affect people varies. TripAdvisor ↗

• 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 ↗

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

Traveler questions