The Centers for Disease Control reported a series of incidents in which pet turtles infected individuals with Salmonella, including one death. All of the incidents were linked to small turtles, which are particularly risky since their size makes them "ideal" gifts to children by well-meaning parents who understand little of the risks they pose.
In the CDC’s MMWR, twenty two cases of Salmonella were reported during the years 2005 and 2006. The fatality occurred in a three month old infant.
According to the CDC, basic hand-washing after turtle contact is a public health necessity (as is the obvious precaution of not letting the turtles get anywhere near food prep areas), The best precaution is not getting the turtles at all.
Source: CDC MMWR – 2007