The aliens died because of all the bacteria and virus in our air, water....etc. We are immune to these organisms or learned to fight them after so many people died from them. The aliens are not immune so they died. This is what I understood from the movie.

that is the funniest part i think. and Tom's son was still alive and back to his grandparents' place earlier than Tom! hahahaha. what a failure..

呵呵，是外星人到了地球上，吃了不干净的东西，喝了不干净的水，

然后拉肚子，又没有黄霉素，拉啊拉啊，就死了。