In my experience, the odd smonkey death is a perfectly normal thing. It's part of the learning process and helps make obvious that something you are doing is wrong. For example, I hadn't fed my smonks quite so much as I should have. As a result, one died and the rest really slowed down. After a few times of trial and error, feeding seemed to liven their spirits and put them on the road to recovery. I'm not saying feed your smonks to make them better, I'm saying that if some are dying it's probably becasue you are doing something wrong. Check temp, location, food, oxgen, that no dirty foreign substances are entering their environment... If it is none of these the smonkey may well have just died of natural causes.
I hope this helps! >:{{{----
Oh right, haha! Sorry for the lecture.
"Problems" seem to be fairly common place in my experience, which is only 1 months worth I might add. I have about 15 or 16 smonks atm and all are healthy except 2. 1 of them has been around since the beginning and I think he is both ill and old now. The other has 2 black dots on his legs, both of which seem to be having no negative effects on him so far, but I'm guessing that they might in the near future. I also had one die about a week ago from having fluff stuck on her and then the formation of a huge black dot on her "neck", the area under her eyes.
So yeah, they are fairly common, but for the most part the majority of your smonkeys should be fit and healthy!