Explain the offset to the physical, economic, spiritual, damage done by illegals killing, raping, drug dealing, robbing, ganging up neighborhoods, flooding the prisons.
Consider what is best for the whole flock, not just a few contagious strays.
During a previous push for amnesty, some columnist found a couple of Vatican papers written by one Fr. Joseph Ratzinger (pre-papacy) that discussed such valid issues when accepting/refusing immigrants.