easiest way would be an .htaccess redirect
RewriteEngine on
Redirect 301 /affiliate1 http://www.affiliate1.com/path/to/whatever?id=112
Redirect 301 /affiliate2 http://www.affiliate2.com/path/to/whatever?id=112
So now any visits to yorusite.com/affiliate1 will redirect to affiliate1.com/path/to/whatever?id=112 .. You could iframe it but some sites break out of frames and it's messy, harder to update... One .htaccess file with your list of redirects and you're set.