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 di