Who hasn't wondered what it would be like to work as a plague doctor -- that secretive scientist who tampers with the biological dark arts, manipulating viral agents to discover new ways of improving delivery, rendering a strain impervious to traditional responses, or selectively tweaking a bug's lethality? It wouldn't be a job for the faint of heart, and I can imagine the paranoia and guilt which accompanied nuclear scientists in the Forties and Fifties might pale in comparison.
I'm not sure if there will ever be a point where human curiosity is trumped by legitimate danger--a threshold that, being too dangerous to cross, incurs a general consensus response of "screw that!" If such an occassion ever does arise, I suspect it will be rooted in the microscopic. The most dangerous things are invariably very small: a split atom, a bundle of seven structural proteins, a stubbornly self-replicating nanobot with a penchant for ecophagy.
Certainly, the plague doctors toe this precipice daily, and for them it's business as usual. I'm not sure I'd have the balls to engage in such work, even if I were to pretend I possessed any inclination for it.
"One scientist from Sandakchiev's Vector Laboratories, Deputy Director Sergei Netesov, appeared one day in 1987 at Obolensk with a new idea for plague: he proposed taking the entire viral genome of Venezuelan equine encephalomyelitis (VEE), perforating the plague cell membrane, and planting the virus inside plague cells like another plasmid. At Vector, Netesov had made a career of proposing such devices. Popov, quoting Alibek, identifies Netesov as the originator of the whole concept of chimeras: genetically engineered viruses made of two component parts---smallpox and VEE, smallpox and ebola. Apparently on the strength of these novelties, Netesov had been promoted to deputy director of Vector at Novosibirsk. The name of the program he directed was Okhotnik---Hunter. His proposed plague-VEE chimera, fiendishly simple in design, but ferocious in concept, is probably the first time anyone had proposed putting together a bacterium and a virus. A victim of this chimera would be treated for plague with the appropriate antibiotics, which would kill the plague bacteria. But shattering the bacterial cell walls would release VEE into the lymph or the bloodstream; the invading virus would have already bypassed much of the immune system, and it would make straight for the brain. Within a week or ten days, the victim would be dead of encephalitis." 
1. Orent, Wendy. Plague: the Mysterious Past and Terrifying Future of the World's Most Dangerous Disease. Free Press, New York: 2004.