What is the difference between myalgia and myositis?

Some similarities : Myositis is actual inflammation of muscles which can occur from certain diseases, for example polymyositis or dermatomyositis. Myalgia is basically pain in muscle which can be caused by many things to include strain, after exercise, illness (like the flu), fibromyalgia, after injection (flu or tetanus shot for example). Myositis will cause pain (myalgia). See doctor if pain not getting better.

Pain vs inflammation: Myalgia means pain in muscles. It may be anything from overuse or excess exercise to a viral infection. Myositis means inflammation in the muscles. This can potentially be a much more serious condition with long term health consequences. You need to be seen by a physician to determine which is present. Myositis is characterized by elevation of muscle enzymes in the blood due to the inflammation.