Some countries allow manufacturers to protect food products using a process called irradiation. The process prevents the rapid alteration of foods, and the resulting propagation of infectious bacteria. Other countries have yet to approve the technique.
Irradiation modifies the molecular structure of foods. As free radicals (very active fragments of molecules) are released new molecules which can be toxic are formed. Irradiation also usually destroys Vitamins B1 and E.
If irradiation is incomplete, the presence of pathogenic bacteria may be masked as more sensitive bacterial indicators are eliminated. The bacteria that causes botulism, for example, is fairly resistant to irradiation, and could continue propagating in meat that has been improperly treated.
We know of no current studies on humans that prove whether irradiated foods are carcinogenic or not, although studies conducted on animals have shown that they can be carcinogenic, or cause undesirable mutations.