Angiostrongylus costaricensis was discovered by Morera and Céspedes in 1971, in a man suffering from an abdominal syndrome. Upon surgery, worms were observed in the cranial mesenteric artery. These worms were assigned to the metastrongylid strongyles and to the genus Angiostrongylus, which was already known, in man medicine, by the species A. cantonensis, the causative agent of an eosinophilic meningitis. Morera's parasite was named Angiostrongylus costaricensis from the place where it was described. A costaricensis is a dixenic parasite, the definitive hosts (D.H.) of which are the cotton-rat (Sigmodon hispidus) and some other rodents. Experimentally, carnivora (Nasua spp: procyonidae) and monkeys (Saguinus mystax: hapalidae) are receptive, same as dogs. The intermediary hosts (I.H.) are slugs belonging to the Veronicellidae family (order gymnophila), mainly Vaginulus plebeius. In the D.H., the parasite produces eggs that hatch into first stage larvae (L1), which are expelled with faeces, eaten by slugs and become infective third stage larvae (L3). L3 are then expelled through mucoïd secretions of the slug and pollute soil and vegetables. D.H. and man get infected with consuming polluted vegetables or even the infected slugs themselves. L3 migrate through lymphatic system and arrive inside the mesenteric artery, where they become adults. In man, the worm can reach this adult egg-laying stage, but larvae are trapped inside granulomas in the intestinal wall and cannot evolve. So, man is a dead-lock for A. costaricensis. Angiostrongylosis costaricensis is an illustration of an hemi-zoonosis (the parasite cannot go back from man to animals) of the biological pattern.