Is it a "real" illness?
No, it's merely a phase or symptom in human motivation processes. It's just as normal as lacking energy after not eating well. However, that does not mean people can't tell you it's an illness to make themselves feel better in some way. Probably financially.
[*]What do/did you do to cure yourself?
There are various ways of dealing with the process. You can study flow and motivational theory and find ways to restructure your life to confirm to society's structure with a lessened presence of senioritis, you can adjust your views on society's life and labor structures and view senioritis as a message from your nature that you want a different pattern in life than society enforces on you, or you can choose for a variety of ways of making living with it more pleasant, or just suck it up.
I don't see it as anything that needs curing so I never did. It simply comes down to the enforced educational and labor structures being boring as hell. It's perfectly natural not to be motivated or do well at things that don't attract you.
You can treat the symptoms though. Turn your work in just in time. Or just enough work to pass. That's what I've done since 2 years before graduating high school and am still doing in my pre-final year at college. This way I do what society demands I do and still have plenty of time and energy left to enjoy the things that actually deserve to be enjoyed.
[*]If you do have "senioritis", is it affecting your programming motivation? If so, positively or negatively? How?
Yes and no. Programming isn't something I do for myself, I do it as work. Yes it slightly affects that, but I'm in a somewhat unique position where my work is entirely up to myself to complete as I wish and has to make way for college and social activities. If it wasn't my work I wouldn't be doing it all, so no it doesn't lessen the amount of programming I do. It's more subject to the structure of tasks in my free time than my motivation to fullfill them.
[*]Is senioritis good or bad?
It's bad if you think it is. I don't see procrastinating for example as anything bad. I just see free time as another task I spend more attention on than essentially useless things like
overperforming at work or school.
Fact is modern life isn't natural so it only makes sense your body and mind oppose it in one way or another. I say, listen to them. I have no need to do anything but not fail college and make enough money to do the things I want to do, which is not making even more money or getting even higher grades.