This article is a travel topic
Mosquitoes are small flying insects that are found throughout the world. In many areas they are primarily a nuisance, buzzing around and leaving itchy little welts where they bite. In other areas mosquito bites pose a serious health hazard for travellers, transmitting tropical diseases such as dengue fever, filariasis, Japanese encephalitis, malaria, West Nile virus, and yellow fever.
Avoid being bitten in the first place: cover your arms and legs as much as possible, and use repellents. The US "Centers for Disease Control and Prevention" (CDC) has recommendations to prevent mosquito bites.
Repellents are available in various forms, including lotions and aerosols that can be applied to the skin, and others that can be applied to clothing.
DEET (diethyl toluamide) - effective at concentrations of 10% and up; its strength plateaus at around 50%. DEET-based repellents are generally the most common, the most effective overall, and are readily available from pharmacies, supermarkets, and vendors at larger airports.
Permethrin - an insecticide used to treat fabrics, mosquito nets, etc (and not suitable for application to the skin). Clothing is usually impregnated by soaking in a permethrin solution; a single application will last several washes.
Picaridin - for those sensitive to DEET, or who dislike its smell, repellents containing Picaridin (e.g. Cutter Advanced) are available in limited areas. These have been shown to be as effective as DEET in certain circumstances, and there's almost no odor.
Oil of lemon eucalyptus (p-menthane 3,8-diol) - another option for those sensitive to DEET or dislike its smell. Considered to be the strongest of the plant-based repellents and according to some studies may be similar to DEET in terms of effectiveness. Has a strong lemon-like odor, but without the acrid chemical edge typical of DEET. Less toxic than DEET.
Mosquito coils, usually permethrin-based, provide effective protection for up to 8 hours in spaces up to 30 cubic metres (a small room). They usually come with little stands: bend the center part upwards, slot the middle of the coil on the pointy bit, heat the other end with a flame until it catches fire, then blow it out and let the end smoulder. The main danger with these is the risk of fire: place the coil on a ceramic plate or other fireproof platform and extinguish it before going out or going to sleep. The smell will also stick to your clothes and other belongings for a while, although it usually fades pretty fast.
Various herbal or plant-based repellents are also available, but their effectiveness is extremely limited.
Sleeping in rooms that are either air-conditioned (so that you can close the windows and doors), or have insect screens over the windows, will help prevent mosquitoes from entering the room. Be sure to check the screens for defects before relying on them. Having a fan switched on can help, as the moving air makes it more difficult for mosquitoes to land.
When camping or sleeping indoors without screens, use a mosquito net that has been treated with permethrin insecticide - these can be found at some pharmacies and airports. As mosquitoes are very persistent insects that will keep trying to find a way to reach you, make sure the net is securely tucked under your bed or sleeping mat.