wah it is quite an old show liao leh...

Any way, I remmebered a bit about that.
They were trying to tell us that we humans have lived in this world for many thousands of years & of coz adapted to this 'unclean' / virus thing. We got immune liao. 'They' alien just came in to our world trying to conqure it, but they dont have the immune thing. The show, or rather the story book is trying to tell us that those alien has destined to fail in the first place.
If I remembered correctly, the bird is attacking the alien, not shit.

And you know why the bird attack the alien? Because the alien is a WORM
