Mosquitoes are common, flying insects that live in most parts of the world. Over 3,500 types of mosquitoes can be found worldwide.Not all mosquitoes bite people or animals. When mosquitoes bite people, the most common reactions to the bite are itching and swelling.Some mosquitoes can be vectors. A vector is an animal, insect, or tick that spreads pathogens (germs) to people and animals. The germs (viruses and parasites) that mosquitoes spread can make you sick.