Hydatid cyst in a lung. Axial computed tomography (CT) scan through the lungs of a 5-year-old girl who has a large (voluminous) hydatid cyst (upper left) in her right lung. Hydatid cysts form in a condition called hydatidosis (cystic echinococcosis). They form around the larvae of a tapeworm (Echinococcus granulosus). Transmission of the larval eggs occurs accidentally from the intestines of carnivores, usually dogs. Humans do not form part of the tapeworm's life cycle. The liver is the most common site of infection, though every organ can be affected. The slow-growing cysts may be surgically removed.
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