I have used the translation "Their defense is weakened by the disease.", and it was rejected. I have made a report to be added.
So can pertahanan also refer to the immune system?
Yes it actually can
Shouldn't the word "oleh" needed between "diperlemah" and "penyakit"? -> "Pertahanan mereka diperlemah /oleh/ penyakit itu". If not, why not?
I believe it can be, but is optional.
This would be a better translation: Pertahanan mereka diperlemah oleh penyakit itu.
I have used the word "Sickness", for "Disease", why is this marked wrong?