The baby didn’t look sick, as he lay in his mother’s arms and looked around the room at the clinic. He wasn’t screaming inconsolably, wasn’t limp and listless.

But his mother had told the nurse that he seemed fussy and wasn’t nursing quite as vigorously as usual. And he felt warm to the touch.

So the nurse took his temperature: 100.4 degrees, less than two degrees above normal.

Paediatricians often need to reassure parents when their toddler or preschooler has a high fever. Yes, we say, your daughter has a temperature of 103, but she looks good; it’s probably just a virus. No need for antibiotics, no need for anything except liquids and an over-the-counter medicine like acetaminophen or ibuprofen.

Yet in this robust baby boy, not quite 2 months old, 100.4 was reason for worry.

Newborns don’t handle infection very well. Their immature immune systems leave them vulnerable to severe infections that can rage out of control.

In the worst cases, bacteria get into the bloodstream, from a urinary infection or a skin infection for example, and cause bacterial sepsis. Or worse, the bacteria leak from the bloodstream through the barrier that is supposed to separate blood and brain, causing meningitis.

So if fever occurs in a very young baby, the advice to parents is always to call the doctor right away. But almost two months old is no longer a newborn.

Twenty years ago, when I was in paediatric training, the definition of “very young” was under 3 months. For any fever in that age group, we took samples of blood, urine and spinal fluid and sent them to be cultured for bacteria.

... contd.