Strengthen your armies against me, my time is now here.
I am the enemy you can't see, I'll fill you with fear.
Carried by those careless, ignorant, and weak.
Keep you captive fairless, suffering that's bleak.
Now don't be afraid, I am mostly innocuous.
Except when my past is weighed, I may be the apocalypse.
Suffer yourselves to be pricked, lest I come at you like a swift kick.
So that you may not be picked, when I'm thrown at you like a brick.
What am I?