3. Candiru

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Don’t be fooled by the benign-sounding name. These nasty little fish that swim in the waters of South American rivers survive by invading the bodies of larger fish, affixing themselves to the gills and feeding off the blood of their host. The problem for people is that they tend to be attracted to any opening they happen to come across when they encounter a person in the water – we think you get the picture! After they have made themselves comfortable inside some hapless human victim, they will then die inside and their decaying body had the potential to cause a serious and sometimes fatal infection.