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Virulence is, by MeSH definition, the degree of pathogenicity within a group or species of parasites as indicated by case fatality rates and/or the ability of the organism to invade the tissues of the host. The pathogenicity of an organism - its ability to cause disease - is determined by its virulence factors. Mosquitoes are in the family Culicidae of the order Diptera and are similar in appearance to other flies. Mosquitoes pass through four distinct stages in their life cycle: egg, larva, pupa and adult. Prodigious numbers of mosquitoes can hatch simultaneously under the proper conditions. In rapidly developing broods, survival of the immature stages can be quite high, but estimates for many species indicate that immature survival is normally less than 5 percent. But 5 percent of millions represents a sizable number. Irrespective of population densities, if they transmit disease or preferentially feed on humans, which many species do, they become appropriate targets for control activities. Malaria deaths in Africa are down by 33% in the last six years, but we must expand access to existing tools and develop new ones to win this fight. Check out Malria No More for more information. In our increasingly globalized world, a single infected person can board a plane and spread a virus across continents. Mark Honigsbaum describes the history of pandemics and how that knowledge can help halt future outbreaks.