You asked for it...

How to introduce a viral/bacterial gene into a plant :

You can't conceivably isolate each and every cell of the plant and painstakingly place the bact/viral gene in each one of them. If you follow this approach, then the most that you could have done after a lifetime of work would be to transform one complete branch or something in that range... Hence, you use another organism to do the work for you... talk about being lazy...

After you read the next section (of delivery of vaccines via viruses, which is very similar) this will seem like child's play to you. However, in short, this is what you have to do:

1) First isolate the gene from the virus/Bacteria you want to immunize against.

2) Next, choose an ideal "TRANSPORT" for sending this gene into the DNA of each and every plant cell. This "transport" is generally a bacterium or virus which infects the plants in nature i.e. without human intervention. The more virulent this transport, the better, since the gene will then be transferred to the plant cells sooner.

3) Now, introduce the gene into the TRANSPORT DNA so that a TRANSGENIC TRANSPORT is formed.

4) The plant is now exposed to the transport (naturally infective bacterium/virus) which infects the plant cells and hence introduces the foreign gene into them.

I'm sorry if I am confusing you here. I got confused when I read it the first time too... :) . Perhaps having a look at the diagram and a description of what exactly Dr. Arntzen is currently doing would help:

Dr. Arntzen's work :

To make a vaccine against a virulent type of E.coli (ETEC) , Arntznen's group plucked a gene from ETEC and spliced (joined) it into a naturally occurring soil bacterium called Agrobacterium tumefaciens. The altered Agrobacterium infected the cells of the potato plants, transferring the foreign gene into them.

The potato cells then started to produce some ETEC protein. When mice ate the potatoes, their immune system recognized the ETEC protein in the pototoes and produced antibodies specific to ETEC.