Myositis is a group of diseases that involve inflammation of the muscles. The muscles become weak and sore because the immune system attacks them by mistake. Myositis can affect the muscles all over the body, making it hard to move or perform everyday activities like walking, getting up from a chair, or lifting things.

There are different types of myositis, but the two most common ones are:

1. Polymyositis: This type affects many muscles in the body, especially those closest to the trunk, like the hips, thighs, shoulders, and upper arms.
2. Dermatomyositis: This type not only affects the muscles but also causes a distinctive rash, usually on the face, chest, back, and sometimes on the knuckles, elbows, and knees.

The main symptoms of myositis include:

• Muscle weakness: This often affects the muscles closest to the trunk (hips, shoulders, thighs, and upper arms) and makes it hard to move.
• Muscle pain or tenderness: The muscles can become sore or uncomfortable.
• Fatigue: People with myositis often feel very tired, even after getting enough sleep.
• Skin rash (in dermatomyositis): A reddish or purplish rash may appear on the face, chest, back, or other areas.

The exact cause of myositis is not completely known, but it's thought to be related to problems with the immune system. Sometimes, infections, medications, or other factors can trigger the immune system to attack the muscles.

Myositis can be diagnosed through blood tests, muscle biopsies, and imaging tests. The treatment often involves medications to help control the immune system, reduce inflammation, and relieve muscle weakness. In some cases, physical therapy may also be used to help strengthen muscles and improve movement.

Although myositis can be a serious condition, with the right treatment, many people can manage their symptoms and lead active lives. In some cases, it may lead to complications, such as difficulty swallowing or breathing, so it's important to get proper medical care and monitor the disease closely.