No one knows why a 12-year-old girl died on Monday. Results from the test to find out whether she had swine flu will come in a day or two.

Only rarely does the flu kill a child, but it does happen. With seasonal flu, about 40 to 60 children around the country die of flu each year. Swine flu, although usually a mild illness, may lead to more children’s deaths because it is infecting more children.

Since Aug. 30, the U.S. Centers for Disease Control and Prevention received 53 reports of childhood deaths blamed on influenza. Of those, 47 were confirmed as H1N1 or swine flu; for the rest, the laboratory could not determine the exact type of influenza responsible. Of the 53 deaths, 43 were in children ages 5 to 17.

Children can die from the flu itself or from a secondary infection, says Dr. Penelope Dennehy, director of the division of pediatric infectious diseases at Hasbro Children’s Hospital. Those who die from the flu alone tend to be children with underlying illnesses, she said.

But about half the children who get severely ill suffer bacterial infections that come on the heels of flu. “Influenza denudes your respiratory tract,” Dennehy says. “It takes away all your normal protection. … That can make you very sick and very quickly.”

Parents should call their child’s pediatrician if the child starts to recover and then gets worse: It could be a sign of a bacterial infection.

Swine flu, like seasonal flu, involves a high fever, dry cough, headache, fatigue, sore throat, runny or stuffy nose, muscle aches and sometimes nausea and vomiting. A child who has these symptoms, and who doesn’t have an underlying health condition, usually just needs to stay home and rest.

But parents should call the doctor if the child shows any of these symptoms:

•Fast breathing or difficulty breathing, a little blue around the lips.

•Dehydration (no urination for eight hours, decreased tears or no tears when crying).

•Severe or persistent vomiting.

•Lethargy or change in mental status (excessive sleepiness, significant decrease in activity level, or diminished mental status.

•Irritability (with infants, does not want to be held or wants to be held all the time).

•Dizziness or lightheadedness.

•Flu-like symptoms improved but then returned or worsened within one to a few days.

•Pain in the chest or abdomen.

Parents should look for symptoms that go “beyond what you would expect from knowing your child and how they behave when they get sick,” Dennehy says. “A lot of what physicians depend on is the parents’ assessment of their child. If a parent is worried, the child feels quite ill, they should not wait.”

But call your pediatrician first. “Our emergency rooms are being overtaxed and in many cases you get better care in the doctor’s office,” Dennehy said.

ffreyer@projo.com