fairly ill/ negative etc

[joham]

I read these sentences in the dictionaries or other books:

The report was fairly incomprehensible. (OALD6, fairly)
I think you'll find it fairly difficult (you do not want to say that it is very difficult). (OALD6, fairly)
lead fairly restricted lives ( OALD, restricted)
fairly negative (OALD, negative)
fairly ill ： AMERICAN ENGLISH TODAY
Measles is still a fairly serious childhood disease in some cuntries. ADVANCED GRAMMAR IN USE, by Martin Hewings

But L.G. Alexander says in his Longman English Grammar that we use fairly before words like good, nice, well rather than those words with a negative meaning. For this reason, almost all the teachers of English in China's classrooms tell their students not to say such phrases as "fairly ill/ cold/ difficult".

Could you tell me how native speakers use this "fairly" before adjectives and adverbs?

[Anglika]

It qualifies them as "not very/not too..." - fairly serious - serious but not too serious; fairly easy - not too difficult.