Myalgia broadly means "muscle pain" and is a symptom of many different diseases and injuries. Myalgia for instance can be used to describe the main symptom of a pulled muscle in which the muscle fibers are ripped in large quantities and this then results in pain and lack of mobility.
Meanwhile myositis means "inflammation" of the muscles. Like myalgia it is not a term that describes a condition but rather one that describes the symptom and likewise it can be a sign of injury, or a side effect from a condition.
What are the causes of myalgia?
In the case of myalgia, the most common causes are injuries such as strains, hematoma, tension etc. Anything that causes a tear or a bruise in the muscle of course will cause pain. This can also occur due to over use – use for long durations (repetitive strain injury) or with too much resistance. Where the condition affects the whole body and chronically then it may be more likely to be an illness.
What are the causes of myositis?
Myositis has similar causes. Most often the cause of myositis is an autoimmune condition, though in some cases it may also be caused by infection that can cause chronic inflammation of the muscles and pain. Symptoms are pain along with weakness and loss of muscle mass. In the case of infection the myalgia and myositis will be a side effect of the infection. This is the case in conditions such as influenza (the flu) but also Lyme disease and many others.