Mild fevers are often beneficial because they represent the body trying to defend itself against infections. Many disease-causing pathogenic microorganisms thrive in a narrow temperature range, so a mild fever prevents them from reproducing. There are many types and models of thermometers meant for different areas of the body. The most appropriate choice is typically determined by the age of the person with the fever — some thermometers are better for small children, for example. Once you've chosen the most appropriate thermometer, using it is relatively straightforward.