[War Of The Worlds]

First off, its a brilliant movie and most of the niggles can be explained,

Why didn't they just take the planet when they buried the tripods?

Easy, there must be alot of the aliens, so I guess they would need a whole load of fertilizer, which means more humans, They were just waiting until there was enough of us. Just like when we wait for a cow to get to its best size before slaughtering it.

Wouldn't they have known about the diseases?

Not really, there must be hundreds of thousands of diseases that are deadly and we don't know about. Some maybe we notice, but can't actually 'see' ir 'detect' Same thing with the aliens, they might not have 'detected' the diseases. Thats the best I can offer on that one because it still baffles me!