A close-up look at the 6 most dangerous micro organisms on the planet. What could these deadly weapons do in the wrong hands?

They say it is not a question of if, but a question of when. They say we’re not prepared. They call it the poor man’s atom bomb – a biological weapon in the hands of a terrorist. The scenario of an attack with such a weapon could be horrific: But how realistic is it? How would it be done? And what kind of counter measures do we have?  
If biological agents are so easy to come by, why have terrorists not used them? If bacteria and viruses can be manipulated so easily using genetic engineering, why has no one invented the ultimate killer bug in their garage? Or have they?