After I learned something In biology I had a scary thought.

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Bacteria casn pick up DNA that is left over from other organisms. So when you think of it, there might be bacteria with bill clinton or GWB dna in them. Some of them might even have Usama DNA!

HAHAHAHA no I don't think so... I learnt this too, and I think that bacteria only take up DNA when the DNA is in solution and there are some enzymes present. Even then, only 1% of the bacteria take up the DNA. So the chance of this happening is tiny!

humans eat food and food is now genetically modified to include genes from non-food sources. so does that mean human cells can pick up non-food dna that the human body has not evolved to handle?

Yúp that is true; two bacteria can exchange DNA.

Bacteria can take up small sections of DNA, true, but remember that these organisms are hundreds of times smaller than eukaryotic cells: there's not enough room in them to take up a whole lot of foreign DNA, even if they've been rendered competent to do so. Usually what they pick up is individual genes; things like bioluminescence or antibiotic resistance.

Gene therapy is the modification of eukaryotic (usually human, or at least mammalian, when used in conversation) genomes. The method being experimented with now involves packaging genes into viral capsids and then allowing the viruses to infect the host; rather than delivering parasitic genomes to their target cells, however, the viruses deliver the corrected version of some gene that the host lacks (say, for insulin production or phenylalanine break down, or whatever). Experiments with humans have been started, but in at least one trial one of the patients died from a severe immune reaction that was thought to be due to the virus used to transfer the DNA.

It is possible to genetically modify a retrovirus which inserts itself into human DNA. The modified retrovirus can be used to carry fragments of human DNA which are then embedded into existing DNA. This was used in gene therapy for some time, but many countries either stopped or scaled back the program after several people died of an extremely rare form of leukemia, assumed to have been caused by the process.

Oh yes I forgot about the retroviruses which can convert RNA into DNA (I think that's the right way around). I learnt about them in AS Biology when doing HIV in Human Health and Disease.