Doctors split respiratory infections into three categories: upper, middle and lower. The upper respiratory infections — infections in the nose, throat, sinuses and ears — include the common cold. Middle respiratory infections include illnesses such as croup, and lower respiratory infections refer to illnesses such as pneumonia and bronchitis.

“Upper respiratory infections usually are minor illnesses even though they may be major nuisances,” says Dr. Susan J. Hubbard, a private practice pediatrician and a member of the medical staff at Children’s Medical Center. “The symptoms often can be managed at home without a doctor visit.” Frequent colds don’t suggest any underlying medical problem, Dr. Hubbard adds.

The main symptoms of upper respiratory infections include fever, cough, runny or stuffy nose, sore throat, loss of appetite, headache, body aches and fatigue. Lower respiratory infections — infections of the airways and lungs — often are more serious. They usually include tightness in the chest, a cough that produces colored mucus, difficulty breathing and fever.

Blame these infections on bacteria or viruses. Bacteria cause otitis media (ear), tonsillitis (tonsils), pneumonia (lungs), bronchitis (airways), sinusitis (sinuses), pharyngitis (throat) and whooping cough (airways). Viruses cause the common cold (rhinovirus), the flu (influenza), pneumonia and even ear infections.

Without tests, it’s tough to tell viral and bacterial infections apart. But in a child with a cold or cough, suspect bacteria (or secondary infection) if a fever comes back after the first few days or the discharge from a runny nose or cough is yellow or green for more than 10 to 14 days. If a sick child starts breathing rapidly (with drooling), has trouble swallowing or seems very irritable or lethargic, you should call the doctor immediately.

Doctors may use antibiotics against bacteria, but antibiotics won’t work on a virus. Children with a virus usually get treatment just for the symptoms. Ask your doctor before giving young children over-thecounter remedies.

Viruses lurk in airborne particles from sneezing, coughing and breathing, as well as surfaces that sick people touch. Infants are most susceptible between the age of 6 months and one year. That’s because the antibodies a child receives from mom are waning, and the immune system still is developing. Make sure children wash their hands regularly — especially after handling things touched by other children or sick people.