Who do you think Diamond's audience is in general for Guns, Germs, and Steel?

This book is written for an audience that is not really
expert in any field, but which is at least interested in the questions that Diamond is
posing.

In this book, Diamond touches on issues that come
from academic fields ranging from microbiology (how and why germs cause the illnesses
they do) to political science (how different types of political organizations emerge). 
This means that the book cannot really be aimed at people who are expert in any given
field the way a book could be if it really just touched on one
subject.

The only real prerequisite for reading this book
(other than being able to read at at least a high school level) is interest.  You have
to be interested in why the world has turned out the way it has and, specifically, why
Europeans and their descendants came to dominate the
world.

So this is a book aimed at anyone with a decent
level of education and an interest in the major issue that the book
explores.