Thought:
"A miniature flying drone modeled after a mosquito with the same size as the mosquito. Either piloted directly via remote control or preprogrammed with flight instructions. Equipped with a piercing hypodermic needle that does not suction blood like its organic version, but rather deploys poisons and venoms directly into the contacts bloodstream. Termination and self destruction post delivery not within contact region. For instances of non obvious termination that leads to suspicion, instead of delivering payload via transmission, delivering oxygen directly to bloodstream to result in palpatations/stroke. Same follow up procedure.

Cons: Targets who are aware of this drone may destroy unaware of what it is.
Drone can be destroyed by unaware or aware environmental factors, not including the target itself.
Price to manufacture for a one time delivery potentially outweighs cost of outcome through said means.

Final thought: Good for espionage work; but not very applicable in regards to warfare. Perhaps application in animal control? (Research further)