Provides monthly checks for people who have worked in the past and who are now unable to work because of a disability. The amount of the monthly disability benefit is based on the Social Security earnings record of the insured worker. SSDI checks may start at the sixth full month of the disability, although claims may be filed before that time.   Claims may also be filed for up to 12 months retroactively.   Individuals with disabilities may qualify for vocational rehabilitation services which could result in a return to work.   Recipients of SSDI may also be eligible for Medicare benefits.   Offers a Compassionate Allowances initiative that benefits people with certain rare diseases.   This initiative will fast-track applications for Social Security disability for applicants with any of 25 rare diseases and 25 cancers whose medical conditions are very severe.   Currently, many people with rare diseases experience lengthy delays when they apply for Social Security assistance because those making the decisions are not familiar with their diseases.