Seasonal Diseases in Thailand

Thailand is a paradise, but every paradise has its pests, and in this case, its pathogens. While the country is generally safe, the tropical climate creates distinct "health seasons" that can catch travelers off guard.

In 2025, Thai health authorities have flagged specific spikes in influenza and mosquito-borne illnesses that are behaving more aggressively than in previous years. Whether you are trekking in the cool mountains of Chiang Mai or island-hopping in the summer heat, knowing which bugs are active can save your holiday.

Here is a breakdown of the most common diseases in Thailand by season and how to protect yourself.

1. The Cool Season (November – February)

The Season of Dust & Flu

While this is peak tourist season due to the pleasant weather, it is also the peak time for viral respiratory infections.

Influenza (The 2025 Spike):

Description: 2025 has seen a significant surge in seasonal flu cases, particularly the A(H3N2) strain. Unlike a common cold, this flu hits fast with a high fever (38°C+), severe body aches, and fatigue.

Prevention: The most effective defense is getting a flu shot before you travel. Wear masks in crowded spaces like the BTS Skytrain or malls.

PM2.5 Respiratory Issues:

Description: Not a virus, but a seasonal hazard. During these months, "temperature inversion" traps pollution over cities like Bangkok and Chiang Mai. This can trigger asthma, bronchitis, and "smog cough" in healthy travelers.

Prevention: Monitor air quality apps (like AirVisual). On "Unhealthy" days, wear an N95 mask outdoors.

2. The Hot Season (March – May)

The Season of Heat & Stomach Bugs

When the temperature hits 40°C (104°F), the risks shift from viruses to environmental and hygiene issues.

Heat Stroke:

Description: This is a medical emergency. Symptoms include hot/dry skin (no sweating), rapid pulse, and confusion. It kills dozens of people in Thailand every year during April.

Prevention: Avoid outdoor activities between 11:00 AM and 3:00 PM. Drink electrolytes, not just water.

Food Poisoning & Diarrhea:

Description: Bacteria grow rapidly in the Thai summer heat. Food left out at markets can spoil in minutes. Common culprits include coconut milk curries and seafood.

Prevention: Eat where the locals eat (high turnover is good). Avoid eating street food that has been sitting in trays; ask for it to be cooked fresh.

Rabies:

Description: While present year-round, rabies cases in animals often spike in the summer. Thailand has a high number of soi dogs (street dogs) and monkeys.

Prevention: Never feed or touch stray animals or monkeys at temples. If bitten or scratched, you must get a rabies vaccine shot immediately (within 24 hours).

3. The Rainy Season (June – October)

The Season of Mosquitoes & Water

This is the most "infectious" season. The humidity and standing water create a breeding ground for vectors.

Dengue Fever:

Description: The biggest threat to tourists. Transmitted by day-biting mosquitoes. Symptoms include "breakbone" muscle pain, high fever, and a rash. 2025 has been a particularly high-risk year for Dengue in the southern islands.

Prevention: There is no cure, only prevention. Use DEET-based repellent during the day (not just at night).

Hand, Foot, and Mouth Disease (HFMD):

Description: Highly contagious among young children. It causes fever and blister-like sores in the mouth and on the hands/feet. It spreads easily in play areas and pools.

Prevention: Wash hands frequently. Avoid public ball pits or crowded indoor playgrounds if you hear of an outbreak.

Leptospirosis:

Description: A bacterial infection spread through water contaminated with rat urine. It is common after floods or walking in muddy water/waterfalls barefoot.

Prevention: Avoid wading through floodwater. If you have a cut on your foot, keep it covered and dry.

Traveler’s Tip for 2025

The "Fever" Rule: If you develop a fever after returning home from Thailand, tell your doctor immediately that you have been to the tropics. Diseases like Malaria or Dengue can sometimes show symptoms weeks after you leave.