The life cycles of the blood and tissue parasites are complex in that they involve both sexual and asexual stages (except for the hemoflagellates such as Leishmania species and Trypanosoma species which have only asexual stages.) Most of the blood parasites are transmitted to humans by an arthropod vector within the sexual phase of the life cycle. Some parasites use insects for development sites and as vectors. Tissue parasites can be either intracellular or extracellular depending on the species and the phase of the parasitic cycle.
The Neglected Tropical Diseases (NTDs) include many blood and tissue parasites and they affect more than 1 billion people in the world in rural areas of low-income countries. The effect of these diseases extends to lost ability to attend school or work, retardation of growth in children, impairment of cognitive skills and development in young children as well to the economic burden of the country. Malaria kills around 660,000 people a year and is difficult to eradicate due to the 30 to 40 species of Anopheles that transmit it, the parasite's ability to evade the mammalian immune system, the use of incorrect drugs to treat it, and other social and environmental factors.
