They used a modified version of the HIV virus to transfer a gene for neprilysin to neurons of genetically modified mice which produced human beta-amyloid.

BBC: Alzheimer's gene treatment hope

The virus used in vaccines is not the actual virus that infects people, but rather a hybrid that's been genetically modified to make it safer, and to give it the ability to multiply more quickly -- a crucial factor when it comes to manufacturing large quantities.

CNN: Candidate virus for H1N1 vaccine arrives at CDC