Towards the end of one particular semester, I had some health issues which forced me to travel in order to get treated. This resulted in me missing the last two weeks of courses and all my final exams. I arranged so that I got grades of "incomplete" in all my courses, until I took the exams and finished my coursework after my recovery. I ended up doing okay, but I did get a B+ in a very important course. It's hard to say whether this was the reason for that, though. Should I mention this on my graduate school application, or will it look like I'm just making excuses?
I think it is worth mentioning. Of course, you don't need to say "my health issues caused a B+". Just mention the health issue at that particular time, and what you had to do to complete your work.
Besides explaining your performance that semester (and a single B+ isn't very much to worry about), this shows your ability to overcome adversity and still succeed.
Here are some other questions about explaining lower grades for non-academic reasons, and the answers largely seem to agree that it is a good idea to mention the circumstances:
In my view, an outlier in the marks is always possible and, unless you apply for a very competitive position, supervisors will account for some slack; often, the most creative people may have blips, and good supervisors know that.
Since you clearly state that you cannot even be sure that the B+ is due to your illness, it may indeed radiate an element of "excuse". However, as dan1111 says, depending on the illness, it may signal your ability to overcome adversity.
Perhaps the cleanest solution is to just mention just factually that you missed your 2 weeks because of the illness, postponed the examinations and retook them after recovery, with the results shown and leave the conclusions to the readers.
There isn't even a need to make excuses for ONE B+ in your undergraduate work -- even in your major. It would be different if you had earned several B+'s in a master's program. Please don't mention it or give it another thought. Just concentrate on celebrating your recovery, and enjoying your studies. And congratulations!