They are hiding among the millions, even learning spanish to try and pass themselves off as mexicans. The FBI director has said as much. Plus, IMHO, the mexicans have decided to wage war against the US of A by their own actions of a criminal illegal invasion, but many refuse to see the truth.
Plenty of people have suggested terrorism as a possible reason for the seemingly endlesss string of food poisonings.
Islamic terrorism is a possibility but I tend to suspect retaliation from the illegals. In either case it fits the description of terrorism in my opinion.