Laos is a tropical country where dengue fever that is caused by mosquitoes is common. Avoid mosquito bites and you will be fine. Bring mosquito repellent with you or buy some in Laos, it is cheap and available in all markets and pharmacies. Wear light coloured clothing with long sleeves and make sure you are covered well at night. Dengue mosquitoes bite in the day, so if you have an afternoon nap, make sure you sleep under a net. In fact nets are a good idea at all times, particularly if you are travelling in the country.