Inclusion Body Myositis (IBM) (a.k.a. sporadic Inclusion Body Myositis, sIBM) is a rare disease (estimated at between 1 and 71 cases per million) but nevertheless is the most common inflammatory muscle disease in older adults.

It has a gradual onset - from months to years - and causes severe muscle weakness in the arms, fingers and legs. It can also lead to difficulties in breathing and swallowing.

The cause is unknown.

There are currently two main theories regarding its origin. Either that its an inflammation / immune reaction (caused by an unknown trigger) or that it's due to accumulation of abnormal proteins in the muscle fibres.

For details see Wikipedia

Note: The name comes from the unusual perforated appearance of microscope slides of damaged muscle fibres, and does not imply that any material is being 'included' in the muscles.