Unskilled in economically useful trades, often unable to understand local language or customs, with no material wealth in their possession, and untold numbers among this throng come bearing only malice toward the host country.
What could possibly go wrong?
It is like a person swallowing larger and larger daily doses of arsenic. Not enough to kill, right away, but the toxic load just keeps building and building. Finally there are organ failures and nerve damage, leading to greatly reduced disease resistance to a wide variety of ills, including cancer, and early death.