The simplest solution would be a simple filter on the_content
:
function my_affiliates_descr($content) {
if (is_single() && has_category('affiliate')) {
$content .= 'This is an affiliate post';
}
return $content;
}
add_filter('the_content','my_affiliates_descr');
You could combine that with category_description()
as suggested in a comment to the question to get:
function my_affiliates_descr($content) {
if (is_single() && has_category('affiliate')) {
$content .= category_description( get_category_by_slug('affiliate')->term_id );;
}
return $content;
}
add_filter('the_content','my_affiliates_descr');