A friend of mine who worked in a greenhouse told me about this and I thought he was full of crap. But it's true.....

Tobacco Mosaic Virus
http://www.extension.umn.edu/distrib...re/DG1168.html

The virus can be spread in many ways to different kinds of plants, but it's common in tobacco plants and can survive in the dried leaves, which is why you have to be careful to wash your hands after smoking.