I believe there is reason for concern w.r.t. modifications of flu viruses, for one thing. That is, they know what makes flu easily transmissible among humans, and they know a bit about how to alter nontransmissible viruses to be more so. Given the belief that things like the 1918 flu are believed to be viruses that made this jump on their own, it's quite reasonable to worry that we could do it in a lab.