I think it can depend on the insurers!!
Bertie became withdrawn Xmas 2005 and went off food and just became very lethargic. Vet investigated everything and they could found nothing wrong and thought that he basically would pass away through lack of food as there was nothing they could do. He had anemia also which they thought was a result of whatever was casuing the problem - which they didn't find out. Then after a wee shot of valium his appetite picked up, then over the next week his blood count improved then he was fine.
Was fine for the next two years - no symptons, no sign of anemia and blood counts all fine. They finally attributed it to a unknown viral infection. They explained that anemia is normally caused by something else, in this case an unknonw infection.
Then same thing happened Christmas 2007 - same symptons and then anemia - again they said it was something unkown. They thought the two instances could not be related as there was two years between them - nothing would go into remission for that long (they said) and again they thought an unknonw factor was casuing the anemia. Eventually after he deterioated his bloods were so bad they said there was nothing they could do.
Tesco didn't pay out the £600 plus in vet fees as this was a pre-exisiting condition they had already paid out for more than 12 months before (they have the 12 month rule!). So despite vet thinking the two things were unrelated because they both resulted in anemia I was not covered!
Lesson learnt for me - I will never use Tesco again!!
Sorry that was rather long winded but just wanted to explain sometimes you never know how it can go!!
The two conditions do not have to be related, if the symptons are the same or similar it may depend on the wording of your vet and your insurer - tesco took 3 months to say no after "doing all they could" !!
. I just gave up as I was so upset at having lost him!