It's possible to conjure up all sorts of terrifying scenarios. What if a GMO gene that causes resistance to antibiotics somehow is transferred from a food into pathogenic bacteria in your gastrointestinal tract? That could make you really, really sick [sources: WHO, Palmer]. But it's probably not going to happen. Dutch scientists, who looked at the risks of specific bacteria being altered by GMO genes in a 2005 study, found that the data "does not give rise to health concerns" [source: Kleter et al.]. And an Australian government scientist concluded in a 2008 study that the frequency of such gene transfers from plant-based foods to microorganisms is so low that it "poses negligible risks to human health or the environment [source: Keese]. Nevertheless, the World Health Organization (WHO) doesn't want to take any chances, and is urging member countries to pick GMOs that don't have antibiotic resistance.
Another potential problem is outcrossing, in which genes from GM plants spread into conventional crops or wild species. Traces of a type of corn only approved for animal use, for example, once began to show up in corn products for human consumption. Some countries have adopted regulations to reduce mixing, including a clear separation between fields with GM and conventional crops [source: WHO]. And a 2006 Australian government report noted that outcrossing could be controlled by making GMO plants' pollen sterile [source: Mills].