This is a really cool story:

"The Dryad's Wedding" by Robert Charles Wilson. It is one of a collection called Year's Best SF 6, edited by David G. Hartwell, published by HarperCollins copywrite 2001.

The story is about people trying to colonize a planet that had never had any mass extinctions. There was no intelligent life, but the entire ecosystem was super-evolved and toxic. The human immune system could not cope and death occured very fast (~5 minutes). But there was intelligence, just a more diffused sort... but I don't want to ruin it for anyone who wants to read it himself.

By the way these collections have a lot of really cool stories in them.