SATURDAY, Jan. 19 (HealthDay News) -- Children often get colds, but when they are not feeling well enough to participate in their normal daily activities or not alert enough to learn or play, they are too sick to go to school, an expert advises.

"Young children's immune systems haven't learned to recognize and resist most common viruses," Dr. Robert Key, a family physician at Mayo Clinic Health System in Prairie du Chien, Wis., said in a Mayo news release. "That's why, until they're 8 or so, kids seem to bring home everything that's making the rounds at school. Children can typically have six to 10 colds per year."

Key added that there are other signs that kids should stay home from school, including:

• Throwing up two or more times during a 24-hour period, or not being able to keep normal foods or drinks down.
• A fever of 101 Fahrenheit or higher.
• Severe coughing or trouble breathing.
• Repeated severe diarrhea for at least a day.
• Stomach pains that last for more than two hours.
• Open sores on the mouth.
• An unexplained skin rash or red eye.

Children who are diagnosed with contagious conditions such as strep throat, chicken pox and impetigo should not go to school until they can no longer pass the condition on to someone else, Key noted.

Colds, the "stomach flu," pink eye and strep throat are the culprits behind most missed school days, Key said. Parents who notice symptoms that seem worse than a common cold should schedule an appointment with their child's pediatrician.

The best way children can stay healthy and avoid missing school is to wash their hands thoroughly and often, Key advised. The U.S. Centers for Disease Control and Prevention encourages people to wash their hands with soap and water for 15 seconds -- about as long as it takes to sing the "Happy Birthday" song twice.

More information

The American Academy of Pediatrics provides more tips to help parents wondering if their child is too sick for school.