(Your question is too general, but somehow, I hope I could help.)
Our body is somehow interrelated to one another. If one organ has a problem, it will try to compensate, but if it reached its limitation, it will trigger the brain that it needs help. Help coming from another part of the body. And another organ/system will try to adapt and compensate. There is also what we call the negative and positive feedback mechanism which our body normally do - It involves other systems so our body can adapt to the changes that occurred due to such disease.
One thing also, if the disease is caused by an infection, other organ or system can be affected because the microorganism or the causative agent that caused the disease can travel through the circulation (in the blood, specifically).
