The parasympathetic nervous system activates the relaxation, or "rest-and-digest," response. Roughly the opposite of the "fight-or-flight" response of the sympathetic nervous system, the relaxation response involves restoration of energy reserves and other "peace time" functions, such as repair and reproduction.
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Roundworms include more than 15,000 known species and up to half a million unknown species. Most do not cause disease, but several species can infect humans, causing symptoms that range from mild to severe. Although serious roundworm infections are uncommon in the U.S.
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Through their transmission of diseases like malaria, yellow fever and West Nile virus, mosquitoes have been responsible for more deaths than any other animal in history.
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