1. What is the cause of baby’s fever? 1.1 Infection The most common cause of fever in babies is infection. Infections can come from a variety of bacteria, viruses and fungi, including influenza, pneumonia, otitis media and diarrhea. When there are foreign pathogens in the baby\’s body, the immune system releases chemicals to fight these invaders, causing fever. 1.2 Teeth Growth When your baby starts to grow teeth, he may also experience fever. This is because the tooth growth process will cause a slight inflammatory reaction, from