The U.S. government warns that ceviche has been implicated in parasitic infection (as well as a vector for transmission of cholera). The high acid content of the lime juice will transform the proteins in the fish as if it were "cooked", but it does not kill bacteria or parasites within the fish's flesh (it may kill some on the surface of the fish). This is dependent upon both the acidity of the marinade and the length of time the fish is marinated. While parasites may be less of an issue with fish purchased from a reputable and modern facility with up to date sanitary precautions, if you're buying from a street vendor in Latin America, you are taking a risk. In any case, from a public health perspective, ceviche is considered "raw fish." The FDA recommends freezing fish intended for raw consumption in order to kill the parasites.
Sources:
U.S. Food & Drug Administration,
Center for Food Safety & Applied Nutrition
FISH AND FISHERIES PRODUCTS HAZARDS AND CONTROLS GUIDANCE: Third Edition. June 2001.
<http://www.cfsan.fda.gov/~comm/haccp4e.html> (25 February 2003)
Robert V. Tauxe, M.D., M.P.H., Eric D. Mintz, M.D., M.P.H.,
Robert E. Quick, M.D., M.P.H., "Epidemic Cholera in the New World: Translating Field Epidemiology into New Prevention Strategies," Emerging Infectious Diseases Volume 1, Number 4, 25 October 1995, <http://www.cdc.gov/ncidod/eid/vol1no4/tauxe.htm> (25 February 2003)