Insects have transmitted diseases since they co-evolved with the hosts and parasites responsible for the disease. An insect that transmits a disease is known as a vector. Insects can act as mechanical vectors, as when house flies transmit viruses or organisms that cause diarrhea. Insects can also serve as obligatory hosts where the disease organism must undergo development before being transmitted (as in malaria). Transmission of disease can take place with the parasite entering the host through the saliva of the insect during a blood meal (e.g., malaria and dengue), or from parasites in the feces of the insect that defecates immediately after a blood meal (e.g., Chagas disease). Parasites transmitted by insects often circulate in the blood of the host, with the parasite residing in and damaging organs or other parts of the body.
In developing countries where insect control is less common, the frequency of diseases is usually greater than in areas with the resources to effectively reduce the populations of disease vector insects.
It is important to remember that while some species of insects are capable of transmitting disease, the vast majority of insects are beneficial to humans.