Sometimes a child's bowel movements are difficult or painful to pass, hard and dry, and occur less frequently than usual. If these symptoms last two or more weeks, your child may be constipated.

Varicella (var-uh-SELL-uh), better known as 'chicken pox,' is a common childhood illness, although it is mostly preventable today after the introduction of a vaccine in 1995.

Some 5 million children have asthma (AZZ-muh), the most common cause of school absenteeism due to illness. Asthma is a condition in which the airways to the lungs constrict in response to irritants.

In the back of your child's throat there are masses of lymphatic (lim-FAT-ick) tissue called tonsils, which produce antibodies to fight respiratory infections.

The most common acute illness in children, especially infants and toddlers, is an upper respiratory (RESS-pur-uh-tor-ee) infection, better known as a cold.

You probably know bacterial conjunctivitis (kun-JUNK-tiv-eye-tis) by the name pink eye. It's a highly contagious infection of the membranes that line the inside of the eyelid and the surface of the eye.

'Croup' (CROOP) is the name given to a variety of infections that involve the vocal cords and the windpipe. When they become inflamed and irritated, they cause a barking type of cough and noisy breathing, especially in children under age three.

Diarrhea (dye-uh-REE-uh) is the presence of watery bowel movements. Frequency can vary, depending how severe the diarrhea is. Acute diarrhea is usually infectious and may be accompanied by other signs of illness, such as fever, nausea, or vomiting, and possibly dehydration (dee-high-DRAY-shun).

An ear infection can involve the outer, middle, or inner ear. Most ear infections in children are the result of a cold virus causing the eustachian (you-STAY-shun) tube to block with fluid in the middle ear.

Fever is one way the body signals it's fighting off infections and other illnesses. It's a warning sign that a child may have an infection. A child's normal body temperature may vary considerably, but it's usually lower in the morning and higher in the evening.

The flu, or 'influenza' (in-flu-EHN-zuh), is a viral infection that typically strikes between late December and March. Symptoms of the flu include: chills, drowsiness, weakness, sudden high fever, headache, muscle aches and pains, coughing, sore throat, and a lack of appetite.

Measles, whose technical name is 'rubeola' (roo-bee-OH-luh), is a highly-contagious childhood disease caused by a virus. The measles vaccine has dramatically decreased the incidence of the disease, although it hasn't totally wiped it out.

Mumps is an acute viral infection of the salivary (SAL-uh-ver-ee) glands, located in front of the ears, which has almost been eliminated by vaccinations.

Otitis Externa (oh-TYE-tis exx-TUR-nuh), otherwise known as swimmer's ear, is an infection of the outer ear. The lining in the outer ear becomes red and swollen, usually due to water entering the ear canal.

Symptoms of a sore throat include pain in the throat, difficulty swallowing, and inflamed throat tissue. There also may be fever, headache, and swollen glands.

'Fever' is the abnormal elevation of the body's core temperature. It's nature's way of fighting off infections and other illnesses. A low-grade fever doesn't necessarily mean something's wrong.

Viruses are micro-organisms (MY-crow-or-gan-izms) that, along with bacteria, cause many illnesses in children. They're a hundred times smaller than bacteria and can't be seen, except with a powerful microscope.

One of the most common causes for abdominal pain in children is the stomach flu, also known as gastroenteritis (gas-tro-in-ter-I-tis). It's usually caused by bacteria or a virus and may cause your child to vomit, have diarrhea, a fever, or all of those symptoms.