What are the dangers of genetically modified viruses?

A virus

Most people never stop to consider the dangers of genetic engineering technique, they just expect it to make pretty babies, correct genetic diseases or create little-eating and fast growing cattle, but men in the street hardly ever take time to look into the technology itself.

The bottom line is that external genetic material is introduced into the host cells (say, GM corn grain) by means of either specially crafted viruses, called viral vector, or bacteria like E-coli that assimilate well foreign DNA, carrying the genes in question precisely because virus has the ability to reproduce and invade other organisms (and bacteria multiply exponentially) — otherwise viruses would be useless.
The increased virulence of E-coli epidemics (and infections in general) in the 20th century could have been caused by such specially crafted bacteria.