In my world, the population of mankind has been all but wiped out by the accidental introduction of extraterrestrial plants brought to earth by scientists who were attacked by them. The plants reproduce by shooting seeds inside the victim at high velocity, and the seeds sprout inside, growing more of the aliens. These creatures can also move around like animals, and they're no bigger than a dog in size. How feasible would it be for such creatures to wipe out most humans?
The technology is that of slightly above modern era, in year 3005. Traveling to different parts of our galaxy is now a thing, although mankind only recently discovered such an ability.