Adults can usually diagnose a fever simply by how rundown and icky we feel, so you don’t necessarily need a thermometer. But an inexpensive digital thermometer (about $10 to $15 at drugstores) is essential if you have kids, says Dr. Hopkins. Opt for the type you can read in 10 seconds if you have small children. Don’t use glass thermometers, which contain toxic mercury your child could ingest if the thermometer breaks. For babies younger than three months, the American Academy of Pediatrics advises calling your pediatrician if your child’s temperature is 100.4 degrees or higher. For all other ages, call your doctor if the temperature is 103 degrees or higher or lasts for more than a day or two, says the American Academy of Family Physicians.