Fever represents the immune system "turning itself on". As it recognizes the arrival of a challenge - usually a viral illness - it signals the production of antibodies and activated T cells against the pathogen. As the 'orchestra" warms up, the fever will rise. One of the substances released, interferon, blocks the ability of viruses to spread, but also makes a person feel 'weak'. Other substances, called "acute phase reactants" have an effect on the intestinal tract. Young children often throw up while older children and adults report nausea or a total lack of appetite.

Different viral infections have different fever patterns. Most rhinoviruses will give a low grade fever, while adenovirus and influenza are famous for high spikes. The height of a fever can also vary with how large a dose of viral particles the person was initially exposed to. Also, if a person had an illness within the previous month, exposure to another fever causing illness will cause an exaggerated fever response. There are many other factors as well. Some families always seem to have high grade fevers.

Special note:

Fever in newborns (infants less than 2 months old) are treated differently than older children. Because the infant's immune system is immature, a low grade fever could signal a serious infection such as a kidney infection or a systemic infection like group B strep. All infants with a rectal temperature over 100.5 need to be taken to the hospital immediately so they can have the necessary tests to rule out bacteremia (bacterial infection of the blood stream). Also, because bacteria in the blood stream of newborns can more easily pass into the spinal fluid, a spinal tap is needed to ascertain that the child is not in the process of developing meningitis.

Most parents are afraid when their child has a fever, especially when the child is young and can't describe what hurts. Moderate and low grade fevers (< 102) are extremely common after six months of age. The child has started to crawl and explore more - putting more things into his or her mouth. Also, after the immediate newborn period, the infant usually has more contact with people - attending family gatherings, music classes, or day care.

Many fevers will start with a high burst in the late afternoon or evening. Parents will often remember thinking that the child acted a "little tired", "whiny" or maybe threw up before the onset of the fever. As a rule of thumb, if a child acted "OK" in the morning and spiked a fever in the afternoon, there is no need to rush to the doctor or hospital right away because this is usually the arrival of a respiratory virus. The child might describe a sore throat, runny nose, or watery stool.

If however, a child has had a low grade fever and cough/cold for several days and then spikes a high fever, there is more of a possibility of a secondary bacterial infection.

If the fever has been high grade (103+) or persistent, the pediatrician will want to see the child to check for serious bacterial infections or autoimmune conditions.

Why treat a fever

If a child is cheerful, or not having pain, it is not necessary to treat a fever. Fever can allow the immune system to work more efficiently. Some pediatricians think that it's better to let the illness "burn itself" out. In many cultures, the thought is that intensifying the fever speeds recovery.

However in children who are not able to express themselves, when a fever is over 103, it is thought that there can be some "vasodilatation" of blood vessels that can give a low grade headache. Bringing the fever down to the 102 range takes away this form of headache. Many children act happy and talkative when they have a fever - some five year olds will even describe vivid dreams that border on hallucinations. It is not necessary to lower fevers if the child is not having pain.

Many viruses make a child have a scratchy throat or throbbing headache. Giving acetaminophen or ibuprofen can eradicate or minimize the pain.

Besides giving fever reducers, other ways to lower a temperature is stroking the child's forehead with a wet hand, or putting cool compresses around the wrists and/or ankles. To illustrate this, put a wet cold piece of cloth on the pulse point of your wrist.

How to take a temperature

For young infants, take a rectal temperature. Place the baby on its abdomen or on its back with knees to chest. Insert a thermometer gently into the anus about 1/4 of an inch. Squeeze the buttocks together to keep the thermometer in place instead of holding it in your hand. . You can remove it in about 30 seconds or when you hear a digital thermometer beep.

Axillary temperatures can be used for toddlers and older children to evaluate for presence of fever, but are generally inacurate. Put a standard thermometer under the child's arm so it touches both layers of skin. Hold it for one minute.

Tympanic thermometers tend to read slightly high if the child is crying. Sometimes the parents will get a 103 or 104 reading when the child is actually 102. Also, there can be an underestimation if there is a lot of wax in the ears, or the child is moving.

Note: to convert Celsius to Fahrenheit, multiply by 1.8 and add 32. To convert Fahrenheit to Celsius, subtract 32 and divide by 1.8.