SSDI is an insurance program for those who are disabled or blind and also who are “insured” by worker’s contributions (you or your spouse have already paid into the program through FICA taxes).
-must have worked for five of the last 10 years
-can only apply once you have stopped working and have to prove that you
became disabled at a time when you were still insured.
-waiting period of five months before you can receive benefits
-You must show that your disability will keep you from working for at least a year. You need to obtain a doctor’s letter stating that you won’t be able to work for one year. However, a doctor’s letter does not automatically mean you will qualify for SSDI.
Source: http://www.ssa.gov/