The short answer is that no-one knows. The presence of inflammatory cells in some muscle samples has led to the hypothesis that muscle is damaged by inflammation, caused by a virus or a misdirected immune system.
It has also been suggested that IBM is primarily a degenerative muscle condition with the deposition of abnormal proteins in the muscle that may trigger an inflammation of the muscles as a secondary part of the condition’s progression. Researchers from around the world continue to investigate the cause of IBM.
Although there are examples of two close relatives getting IBM, e.g., siblings or parent-offspring, this is extremely rare. Generally, IBM is considered an acquired rather than a genetic condition so it is very unlikely that more than one person in a family will have IBM.