Mosquitoes in Thailand: the stop-by-stop briefing
Bangkok's street-food lanes, Chiang Mai's temples, a week on Koh Lanta: the same day-biting mosquito follows you everywhere, while malaria stays in forests most tourists never see. Here's what changes from stop to stop, when dengue peaks, and what actually needs a clinic.
Dengue is the Thailand mosquito story: everywhere, in every month, peaking in the rainy season (roughly June to September), carried by a mosquito that bites in daylight. That means repellent over sunscreen every morning plus screened or air-conditioned sleep. Malaria pills are not for the usual route: CDC rates Bangkok, Chiang Mai, Phuket and Koh Samui minimal-risk and reports no transmission at all on Krabi's islands or in Pattaya. Only rural forest along the Myanmar, Cambodia and Malaysia borders needs a clinic visit. Pregnant? Zika circulates at low levels; talk to your provider first.
Thailand is where a lot of travelers meet tropical mosquitoes for the first time, and the classic mistake runs in both directions: packing malaria pills for a Bangkok-and-beaches trip that doesn't need them, then skipping repellent in the daytime because "mosquitoes come out at night." Thailand's real risk works the other way round. The dengue mosquito, Aedes aegypti, keeps office hours and lives in cities, and in recent years it has been busier than ever.
The departure board
| Code | Stop | Dengue | Malaria (CDC) | Also on the board |
|---|---|---|---|---|
| BKK | Bangkok | Year-round, urban | Minimal risk | No JE vaccine for a short city trip |
| CNX | Chiang Mai · Chiang Rai | Year-round | Minimal in the cities | JE season May–Oct in the northern valleys; chikungunya flared in 2025 |
| HKT | Phuket · Koh Samui · Koh Pha Ngan | Year-round | Minimal risk | A traveler's Zika infection was traced to Phuket in 2024 |
| KBV | Krabi islands · Pattaya | Year-round | None | Koh Lanta, Koh Phi Phi, Koh Yao Noi, Koh Yao Yai |
| TDX | Koh Chang · Koh Mak · Koh Kood | Year-round | Ask a clinic: Trat borders Cambodia | Biting midges on quiet beaches |
| MAQ | Border forests: Myanmar, Cambodia, Malaysia | Year-round | Prophylaxis area | Treks, jungle lodges, rural stays: clinic 4–6 weeks out |
Malaria column: CDC Travelers' Health, Thailand (reviewed October 2026). CDC names the prophylaxis area as provinces bordering Myanmar, Cambodia and Malaysia, especially rural forest and forest-fringe areas.
Thailand's dengue years, from our own data
We track dengue for every country in our briefings from official case counts, so this chart isn't a press-release estimate: it's Thailand's reported cases, year by year, from the OpenDengue database.
Reported dengue cases in Thailand, 2012–2024. Source: OpenDengue (Clarke et al., Scientific Data 2024), V1.3, CC BY-SA. The dashed line is the 2015–19 average.
Three things stand out. 2023 was the biggest year in the series, with about 159,000 reported cases, and 2023–24 together averaged 1.76 times the pre-COVID baseline. Dengue moves in waves: 2021's tiny number likely reflects COVID-era testing and travel as much as mosquitoes, and big years tend to follow quiet ones. And the wave is currently receding: WHO counted 57,440 cases in 2025, and Thailand's Department of Disease Control reported 21,620 by 19 August 2026, below 2025's pace. A quiet year is good news, not a reason to skip the morning coat.
When: the rain is the calendar
Thailand's rainy season runs roughly mid-May to mid-October (the south stays wet until December); Thailand's Department of Disease Control puts the dengue peak around June to September. Cases are reported in every month.
Whatever the month, the routine is the same: repellent at breakfast, over sunscreen, topped up after sweat or a swim, and again before the late-afternoon peak, when rooftop bars and night markets fill up. The full hour-by-hour version is our dengue-zone daily routine, and the duration calculator tells you how long your bottle really lasts in that humidity.
Malaria: a border story, and a rebound
Thailand nearly eliminated malaria. WHO's estimates in our data show cases falling from about 47,000 in 2012 to about 2,400 in 2021, before rising again to roughly 9,000 in 2023, a resurgence public-health groups link to the conflict in neighboring Myanmar; WHO's Mekong program counted 14,688 confirmed cases in 2024, almost half of them imported. Almost all of that sits in rural forest and forest-fringe villages along the Myanmar, Cambodia and Malaysia borders, which is exactly where CDC draws its prophylaxis line.
The border clause. If your plan includes trekking, jungle lodges or rural homestays in border provinces (Tak and Kanchanaburi toward Myanmar, the eastern provinces toward Cambodia, the far south toward Malaysia), book a travel clinic 4–6 weeks before departure with your exact route. CDC lists atovaquone-proguanil, doxycycline and tafenoquine as options there; which one, if any, is a professional's call. Our malaria-zone checklist covers everything around the prescription.
The other three: Zika, chikungunya, Japanese encephalitis
Zika circulates at low levels. Thailand's Department of Disease Control counted 758 cases in 2023, up from 190 in 2022, European tourists among them, and CDC's journal described a pregnant traveler infected in Phuket in 2024 whose fetus developed microcephaly. There's no CDC Zika notice for Thailand, but CDC's advice for pregnant travelers to places with current or past Zika is to talk to a healthcare provider before going. Our babies, kids and pregnancy guide has the details, and the age rules if your travelers are small.
Chikungunya, another day-biter's disease, is on CDC's elevated-risk list for Thailand; a 2018–19 outbreak reached about 15,000 cases across 60 provinces, and 2025 ran above recent years in the north and northeast. CDC suggests considering the vaccine only for long stays (six months or more).
Japanese encephalitis is present across much of Thailand outside Bangkok, with northern epidemics from May to October. CDC recommends the vaccine for stays of a month or more in affected areas, suggests considering it for shorter trips with rural, camping or unscreened nights, and doesn't recommend it for a short city break. And dengue itself? There's no dengue vaccine available in the US; Qdenga is authorized in Thailand and dozens of other countries, and WHO says travelers who've already had dengue are the ones most likely to benefit, which is a travel-clinic conversation, not a pharmacy purchase.
The island footnote: "sandflies" that aren't. On some quiet beaches, Koh Chang, Koh Mak and Koh Kood among them, the tiny biters at dawn and dusk are biting midges, which Thai beaches call sandflies. They're a nuisance, not a disease risk, and they play by the rules in our Barbados sandfly guide: repellent aimed at the ankles, lounging on the breezy side of the beach, and a fabric layer at sunset.
Buying repellent in Thailand
You won't run out. Soffell, a low-strength DEET lotion (around 12–13%, depending on the version), sits next to the till at nearly every 7-Eleven and pharmacy, alongside OFF! and Sketolene, whose range includes a 20% DEET spray. Low-strength DEET works; it just needs re-applying more often in Thai heat. Picaridin is the harder find: larger Boots and Watsons branches are the best bet, and there are Thai-made icaridin sprays, but don't count on them outside big cities. If picaridin is your preference, or you want 20% strength for long days, bring it from home: a 3.4 oz (100 ml) bottle rides in your carry-on. Here's how picaridin and DEET compare on the road.
The Thailand kit
- Picaridin 20% travel sprays: odorless in Bangkok heat, layers over sunscreen, and won't damage sunglasses or straps. One 3.4 oz bottle covers about a week of daily use.
- Screened or air-conditioned rooms, plus a plug-in vaporizer from any 7-Eleven for open-air bungalows.
- Permethrin for trekking clothes: border-forest and jungle-lodge itineraries only, treated at home 48 hours before the flight.
Traveler questions
Do I need malaria pills for Thailand?
Not for the usual tourist route. CDC lists Bangkok, Chiang Mai, Chiang Rai, Phuket, Koh Samui and Koh Pha Ngan as minimal-risk, with no transmission on Krabi's islands (Koh Lanta, Koh Phi Phi, Koh Yao Noi, Koh Yao Yai) or in Pattaya City. Prophylaxis is for rural forest areas in provinces bordering Myanmar, Cambodia and Malaysia; if your route goes there, see a travel clinic 4 to 6 weeks before you fly.
When is dengue season in Thailand?
The rainy season, roughly mid-May to mid-October, with cases peaking around June to September; the south stays wet until December. Dengue is reported in every month, so a dry-season trip still needs daytime repellent.
How bad is dengue in Thailand right now?
2023 was Thailand's biggest dengue year since at least 2012, with about 159,000 reported cases, and 2024 still had about 105,000. WHO counted 57,440 cases in 2025, and 2026 has run lower again, with 21,620 cases by 19 August according to Thailand's Department of Disease Control. Dengue moves in multi-year waves, so a quiet year is not a reason to skip repellent.
Is there Zika in Thailand?
Yes, at low levels. Thailand's Department of Disease Control counted 758 Zika cases in 2023, up from 190 in 2022, including European tourists, and a pregnant traveler infected in Phuket in 2024 was described in CDC's journal. There is no CDC Zika travel notice for Thailand, but CDC advises pregnant travelers to areas with current or past Zika transmission to talk to their healthcare provider first.
Do I need the Japanese encephalitis vaccine for Thailand?
CDC recommends it for travelers spending a month or more in areas with Japanese encephalitis, suggests considering it for shorter trips with higher-risk activities such as rural stays, camping or sleeping without air-conditioning, screens or nets, and does not recommend it for short trips to cities.
What mosquito repellent can I buy in Thailand?
Soffell, a low-strength DEET lotion, is at nearly every 7-Eleven and pharmacy, alongside OFF! and Sketolene, whose range includes a 20% DEET spray. Picaridin is harder to find; larger Boots and Watsons stores are the best bet. If you prefer picaridin or want 20% strength for long days, bring it from home.
Sources: CDC Travelers' Health and Yellow Book pages for Thailand (reviewed October 2026); WHO SEARO epidemiological bulletins; WHO Mekong Malaria Elimination Programme (July 2026); Thailand Department of Disease Control; OpenDengue V1.3; WHO Global Health Observatory malaria estimates. Health information summarizes official sources and is not medical advice; see a travel clinic for vaccines and prophylaxis.