Use our guide to treat Baby--whether she has a fever or the flu. We'll help you decide when to push the worrying aside, and when to call the doctor.

Fever

This is a sign of an underlying problem rather than an illness on its own. "The body raises its temperature because the enzymes that fight infection work better at higher temperatures," Dr. Horowitz says. A fever can be related to an ear infection, a cold, the flu, or it can be a reaction to a vaccine. Feverish kids are lethargic and irritable.

How to treat: Some parents have "fever phobia" and want to take their kids to the doctor for the slightest increase in temperature. But low-grade fevers (101 degrees or below) typically aren't dangerous. Give baby Infants' Tylenol, undress her, encourage her to drink fluids, and bathe her in tepid water. "Just wiping her down could lower her fever by a degree or two," says Christopher Tolcher, MD, a pediatrician in Agoura Hills, California.

When to call the doctor: If your baby is a newborn (2 months or younger) and has even a low-grade fever, if your child is younger than 1 year and has a fever of 102 degrees or higher, if an older child has a fever of 104 degrees or higher, or if the fever (even a low-grade one) lasts more than three days. Drastic changes in behavior--such as your child's becoming very lethargic--are also cause for concern.

The Common Cold

'Tis the season for sniffling. Young children get six to eight colds a year, primarily during fall and winter. Symptoms (a stuffy or runny nose and sneezing, coughing, and sometimes a fever) usually build for two to three days, peak for three to five, and then subside.

How to treat: Avoid cough or cold meds, but give him Infants' Tylenol for fever. Dry air worsens congestion, so run a humidifier in baby's room while he sleeps. You can also drain his sinuses by elevating his head. Place books or pillows under the mattress while he sleeps; don't put pillows directly under his head. If he's not drinking lots of breast milk or formula, give him water or an electrolyte drink, like Pedialyte.

When to call the doctor: If your sniffling baby is a newborn, or if he has a high fever.

The Flu

This very contagious virus travels quickly through day cares and families. An infected baby will be cranky and will lose interest in playing or eating. Then she'll develop a fever followed by a runny or stuffy nose and a cough. A flu-related fever can last three to seven days, and kids can continue to feel cranky for a few days after that.

How to treat: Care for your child as if she has a cold: give her plenty of liquids and watch out for severe coughing or breathing problems. To prevent a future bout, get your child vaccinated, which experts recommend for all kids over 6 months.

When to call the doctor: If baby is a newborn and sick, or if symptoms don't improve within five days.

Respiratory Syncytial Virus (RSV)

This sounds scarier than it is. It's a common virus affecting the lungs and breathing passages. Preemies are especially vulnerable because their airway and immune system aren't completely developed. It generally begins like a cold does; by day three, a strong cough and wheezing-like breathing begin. Symptoms recede after a few days, but the cough can linger for up to two weeks. Kids who have a bad episode tend to develop asthma.

How to Treat: Use a humidifier, or have baby sleep in an elevated position. Also give him Infants' Tylenol and extra fluid. Cool air can calm irritated airways: place him in front of an open fridge for a minute.

When to call the doctor: If your child is a newborn and sick, or if he has severe nonstop coughing or trouble breathing. "Notice if he's using extra muscles to breathe," Dr. Horowitz says. "Take off his shirt. If the spaces between his ribs get sucked in with each breath and his nostrils are flaring, call your doctor right away."