Learn how to interpret the readings of the thermometer
Keep in mind that newborns have lower than normal body temperatures compared to adults — typically less than 97 °F, versus a normal of 98.6 °F in adults. Thus, a temperature reading that indicates a mild fever in adults (100 °F or 37.8 °C, for example), could be more significant for a baby or infant. Furthermore, the different types of thermometers have slightly different ranges of normal because they measure body heat from different locations. For example, your child has a fever if they: have a rectal or ear temperature reading of 100.4 °F (38 °C) or higher, an oral reading of 100 °F (37.8 °C) or higher, and/or an armpit reading of 99 °F (37.2 °C) or higher.
In general, contact your doctor if: your baby (3 months or younger) has a rectal temperature of 100.4 °F (38 °C) or higher; your infant (three to six months of age) has a rectal or ear temperature greater than 102 °F (38.9 °C); your child (six to 24 months of age) and has a temperature reading over 102 °F (38.9 °C) on any thermometer that lasts longer than a day.
Most healthy adults can tolerate fevers as high as 103 – 104 °F (39 – 40 °C) for short periods of time without having problems. However, temperatures between 105.8 – 109.4 °F (41 – 43°C), termed hyperpyrexia, are serious and require medical attention. Temperatures above 109.4 °F (43 °C) are almost always fatal (deadly).