There are various forms of disability insurance. Some are provided by your employer (or you can purchase your own policy) and then there are Social Security Disability Insurance Benefits (DIB). Short term disability insurance is provided by your employer and covers a short period of disability. Long term disability insurance, as the name implies, covers a longer period of disability. Usually, the amount of money you receive from long term disability decreases as times goes on. The amount of reduction is based the policy you have.
If you become unable to work because of illness, you should apply for Social Security Disability as soon as possible. It can take 1.5+ years to attain Social Security Disability Benefits. During this time your short term disability will run out and your long term disability may be reduced. Many insurance companies require you to file for Social Security Disability if you are receiving long term disability.
Another reason to apply for DIB is that you must be found disabled before your date last insured (DLI) to receive the benefit. If you wait too long to apply for DIB your DLI may be in the past and you will have a more difficult time proving your disability started before that date.
If you are receiving long term disability and are approved for Social Security Disability, your insurance company may require you to repay some of the money you received in long term disability. You should inform your long term disability insurance carrier as soon as possible that you have been approved for Social Security Disability Insurance.