Viral myositis (my-uh-SYE-tis) is inflammation (pain and swelling of the muscles). It can happen while a child is sick with a virus or 1–2 weeks later. Kids may have muscle pain, weakness, and sometimes swelling in the calves. With rest and fluids, most children feel better within a few days.

Use these instructions to care for your child.

• Offer your child lots of liquids to drink.
• Encourage your child to rest when they feel weak or tired.
• If your child has pain, a medicine may help. You can give acetaminophen (such as Tylenol® or a store brand). Follow the package directions for how much to give and how often.
• Do not give any other medicines to your child without checking with your health care provider first.
• Do not give aspirin to your child or teen as it is linked to a rare but serious illness called Reye syndrome.
• Follow the health care provider's instructions about when your child can return to sports and active play.

Your child has:

• a new or higher fever
• muscle pain, swelling, or weakness that gets worse or spreads to new areas of the body
• muscle pain or weakness for more than 3 days
• leg pain on only one side
• new symptoms

Your child:

• has red or brown urine (pee)
• appears dehydrated. Signs include dizziness, drowsiness, a dry or sticky mouth, sunken eyes, crying with few or no tears, or peeing less often.

Why does myositis happen after some viruses? Health care providers aren't sure why muscle problems sometimes happen after someone has a virus. It could be due to the way the body fights the virus or the virus getting into the muscles.

Which viruses are the most likely to cause muscle pain in children? Influenza A and influenza B are the most common viruses that cause muscle pain. With the influenza viruses, the pain is almost always in the calves. The pain can be strong enough to cause a child not to walk.