The one change Spielberg made from the book to the movie, was one dumb change, by having the aliens already here.
How would earth bacteria kill them, if they had already been on earth for so long?
What, a super hyper delayed reaction?
The book made sense, since they came from space, and didn't know about the bacteria, and after it kicked in, it affected them, in the movie, they have already been here, are here, and now it kicks in when they start doing stuff.
I get the reason why he changed it, but it was a stupid reason.
The aliens rode down in metal capsules inside lightening bolts.
The tripods were already here....not the martians.