Viruses have their own genetic material and can infect human cells in a very specific manner. They will then reproduce as directed by their own genes but using the resources of the host cell. These properties make them interesting “gene shuttles” to fight hereditary diseases or cancer. There are innumerable different viruses, but the human adenovirus 5, which normally causes the symptoms of a typical cold, has substantial advantages: Its genome can be replaced completely by an artificial one which contains only “useful” genes. Without any of the viral genes left, the virus can no longer replicate and trigger diseases. In addition, the genome of the adenovirus is very large and does not integrate into human chromosomes.