An example of a nasty trick that would make for a relatively easy to produce and deploy bio-weapon : http://www.plospathogens.org/article/info%3Adoi%2F10.1371%2Fjournal.ppat.1001257
Inhaled prions have extremely long incubation times (years), so it would be possible for an attacker to expose huge numbers of people unknowingly to them. The disease it causes is slow and insidious, and as of today, there is no way to detect it until post-mortem. There's no treatment, either. I'm not certain of the procedure for making prions in massive quantities in the laboratory, but since they are self-replicating if placed in a medium containing the respective protein, they probably could be mass-produced.
On the bright side, the disease would not be self-replicating in the wild, so it would not be an existential risk - merely a very nasty way to cause mass casualties. Also, this method has never been tested on humans, so it might not be very effective, so one can hope that terrorists will stick with bombs.
The FHI's mini advent calendar: counting down through the big five existential risks. The second one is a new, exciting risk: synthetic biology.
Synthetic biology
Current understanding: medium-low
Most worrying aspect: hackers experimenting with our basic biology
Synthetic biology covers many inter-related fields, all concerned with the construction and control of new biological systems. This area has already attracted the attention of bio-hackers, experimenting with DNA and other biological systems to perform novel tasks – and gaining kudos for exotic accomplishments. The biosphere is filled with many organisms accomplishing specific tasks; combining these and controlling them could allow the construction of extremely deadly bioweapons, targeted very narrowly (at all those possessing a certain gene, for instance). Virulent virus with long incubation periods could be constructed, or common human bacteria could be hacked to perform a variety of roles in the body. And humans are not the only potential targets: whole swaths of the ecosystem could be taken down, either to gain commercial or economic advantages, for terrorist purposes, or simply by accident.
Moreover, the medical miracles promised by synthetic biology are not easily separated from the danger: the targeted control needed to, for instance, kill cancer cells, could also be used to target brain cells or the immune system. This would not be so frightening if the field implemented safety measures commensurate with the risks; but synthetic biology has been extremely lax in its precautions and culturally resistant to regulations.