Mosquito bites can cause severe skin irritation for some people through an allergic reaction to the mosquito's saliva. The mosquito is an important vector for several disease pathogens to humans and animals, such as, protozoans (Malaria), nematodes (heartworm) and arthropod-borne viruses (Yellow Fever, Dengue Fever, West Nile, St. Louis Encephalitis, Eastern Equine Encephalitis, etc.).
A blood fed mosquito. Source: Multnomah County Health Department
Arthropod-borne viruses: Arboviruses are the most diverse, numerous and serious diseases transmitted to humans and other mammals by vector mosquitoes and other blood-sucking arthropods. All arboviral encephalitis are maintained in complex life cycles involving a nonhuman primary vertebrate host and a primary arthropod vector. Humans and domestic animals can develop clinical illness but usually are dead-end hosts due to low viremia (low levels of virus in the bloodstream) produced by them and do not contribute to the transmission cycle. Many people may not even know they are infected with an arbovirus. When symptoms do occur (2-15 days from the bite of an infected mosquito), they may include fever, headache, fatigue, dizziness, weakness and confusion. West Nile virus may also cause rash or muscle weakness. The most severe cases can lead to coma and death. There are several virus pathogens of encephalitis in the U.S. : West Nile Virus (WN), Eastern Equine Encephalitis Virus (EEE), Western Equine Encephalitis Virus (WEE), St. Louis Encephalitis Virus (SLE), La Crosse (LAC) Encephalitis, Dengue Fever (DEN) and Yellow Fever are transmitted by vector mosquitoes.