From the Bookshelf of Apocalypse Whenever…

[close] November Book, read anytime before November 1st, when we will start discussing it.
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Some biologists speculate that if we ever make contact with extraterrestrials, those life forms are likely to be--like most life on earth--one-celled or smaller creatures, more comparable to bacteria than little green men. And even though such organisms would not likely be able to harm humans, the possibility exists that first contact might be our last.

That's the scientific supposition that Michael Crichton formulates and follows out to its conclusion in his excellent debut novel, The Andromeda Strain.