On one hand, immigration is great. The Irish weren't well liked because there was a huge influx of them all at once, but what would we do nowadays without our cosy Irish bars and pints of Guiness?! We'd be lost. On the other hand, genocide. So, hmmm.
On the other, other hand, if aliens invaded tomorrow, we'd probably forget about cultural barriers altogether and really come together as humanity. Of course, the wait to see a doctor would still be quite long.