Mosquitoes can act as a vector for many disease-causing viruses and parasites. Infected mosquitoes carry these organisms from person to person without exhibiting symptoms themselves. Mosquito-borne diseases include:

Viral diseases, such as yellow fever, dengue fever and Chikungunya, transmitted mostly by Aedes aegypti. Dengue fever is the most common cause of fever in travelers returning from the Caribbean, Central America, and South Central Asia. This disease is spread through the bites of infected mosquitoes and cannot be spread person to person.

Severe dengue can be fatal, but with good treatment, less than 1% of patients die from dengue.

• The parasitic disease malaria, carried by mosquitoes of the genus Anopheles

• Lymphatic filariasis (the main cause of elephantiasis) which can be spread by a wide variety of mosquito species
• West Nile virus is a concern in the United States, but there are no reliable statistics on worldwide cases.
• Tularemia, a bacterial disease caused by Francisella tularensis, is variously transmitted, including by biting flies. Culex and Culiseta, are vectors of tularemia as well as arbovirus infections such as West Nile Virus.

Though originally transmission of HIV was a public health concern, practical considerations and studies of epidemiological patterns suggest that any transmission of the HIV virus by mosquitoes is in practice extremely unlikely at worst.

Various species of mosquitoes are estimated to transmit various types of disease to more than 700 million people annually in Africa, South America, Central America, Mexico, Russia and much of Asia, with millions of resultant deaths. At least two million people annually die of these diseases, and the morbidity rates are many times higher still.