Fever in children is of great concern to parents because it can indicate something serious.
But this may be difficult to discern in young children, as they cannot report what they are feeling.
Also, there is much misunderstanding about what you should or should not do for a child with fever.
Studies have shown that the perception of fever varies widely, even among doctors and nurses. This can increase parents' anxiety.
As human beings have body temperatures ranging between 36 and 37.5 deg C when they are well, any reading above 37.5 deg C should be considered abnormal.
Many doctors consider temperatures between 37.5 and 37.9 deg C as "low-grade fever".
Doctors and experienced parents usually consider four key factors: the child's age, how long the fever has been going on, the source of the fever and whether the child is functioning well.
The first factor recognises that very young infants may be at the highest risk of complications, such as death, if they have a serious illness. That is why the rule of thumb is that any infant up to one month old who has fever is hospitalised, investigated and treated.
The second point is based on the age-old saying that "with time, everything becomes clear".
The cause of the fever may not be clear to a doctor who sees the child on the first or second day of fever, when clues that doctors are trained to look for may not have appeared yet. They include a specific kind of rash, swelling of the fingers or enlargement of the liver and spleen.
These clues may appear by the fifth day, or even later. So, a child with fever lasting five days or more should be reviewed by the doctor.
The third recognises that the common cold, which is often accompanied by a fever that starts on the same day as the nose symptoms, is almost always due to viruses, which do not require specific treatment. However, symptoms unrelated to the common cold need to be assessed, as they can be caused by more serious problems.
Finally, children who have serious illnesses may be lethargic, drink very little and pass urine only once or twice a day. If such symptoms persist over most of a day or more, that is a red flag.
The pattern of fever and sequence of symptoms can tell doctors whether they are dealing with a more serious problem or not.
I call this the "timeline".
Each disease, even a rare one, has a specific timeline and therefore can be suspected early on if the pattern fits its profile.
One example of a fever timeline is the "infection pattern". Here, symptoms that indicate the main source, like a cold, start first.
The fever then starts around the same day, or within the next 24 hours of the source symptoms.
As the child recovers, the fever disappears first. The source symptoms - such as those of the cold - are the last to subside. This holds true for almost all infections.
Rare illnesses, such as cancer and inflammatory disorders, as well as several serious bacterial infections usually do not follow the classic infection pattern. Recognising this is important for the early diagnosis of serious illnesses.
A recent example is that of an eight-month-old girl who came into the children's emergency department with fever and diarrhoea.
She was diagnosed with a viral infection of the intestine. When she returned for a review on day four, the diarrhoea had disappeared, but the fever was still there.
Mental alarm bells went off.
She had not followed the classic infection pattern - the fever should have disappeared first. This means she almost certainly did not have a viral infection of the intestine.
As there were no other clues at that time, I told the parents to take her in again in the next few days if the fever persisted. They diligently took her back a few days later. Then, she exhibited a faint redness of the eyes. Blood tests were performed and she was hospitalised.
The baby girl had Kawasaki disease, which is an uncommon inflammatory disorder that, if untreated, could result in long- term heart problems.
Even while hospitalised, she had only a few, subtle signs of Kawasaki disease. She received prompt treatment and responded as expected.
Three days later, she was discharged, well again.
•Dr Soh Jian Yi is a consultant in the division of paediatric allergy, immunology and rheumatology at National University Hospital.