Which term means a disease characterized by the simultaneous inflammation of voluntary muscles in many parts of the body?

polymyositis

Explanation:

Polymyositis is the disease in which inflammation of voluntary muscles and associated tissue occurs in various body parts. So it affects both sides of the body. Most of the weakness is seen in muscles of the shoulder, upper arm, thighs, neck, and hips.  

This condition makes some jobs difficult for patients like difficulty in climbing stairs or lifting objects, etc. Women are at higher risk for this disease than men. Till now it's cure is not available but several therapies and physical exercise can help in gaining muscle strength.

that is wrong, its not thymus.