Your car breaks down. The mechanic finds it and fixes it . You can also do preventive maintenance so it's less likely to break down. Right?

Sometimes genes are defective. Certain genes or bits of DNA cause certain diseases. If you can find and remove the defective gene you don't get the disease. Is that factual or theoretical? If factual what would happen if you got the disease THEN removed the cause..the defective DNA? Would the disease go away? Why couldn't it work that way?