At that age, physicians are a little quicker to treat with antibiotics, even when we're not sure it's a bacterial infection. In infants, an abnormally low temperature can also be a sign of illness. Also, a child that is inconsolable, has a stiff neck or is hard to awaken may be ill. Parents are the best judges of their baby's normal behavior, and an alert parent can often spot a problem before serious symptoms develop. Children older than three months handle infections better, so a mild fever is less of an immediate need.
Fever makes children uncomfortable, even in a mild illness.
If you give them acetaminophen or ibuprofen to lower the temperature, and they still feel sick 45 minutes later, call immediately. If any child has breathing difficulties, at any age, that is a cause for immediate action. Fast breathing or labored breathing could be asthma or croup, and needs to be seen immediately. I generally recommend the guides put out by the American Association of Pediatricians. Both are available at local bookstores and combine practical medical advice with other parenting information and a guide to normal child development.