Viral Infection: This is the most common cause of fever in children of this age group, often due to self-limiting viral illnesses such as upper respiratory tract infections.

Bacterial Infection: Although less common than viral infections, bacterial infections (e.g., urinary tract infections, pneumonia) can present with fever alone initially.
Teething: In some cases, teething can cause a low-grade fever in infants and toddlers.
Immunization Reaction: Recent vaccination can lead to a low-grade fever as a common side effect.

Meningitis: Although rare, meningitis is a life-threatening condition that can initially present with fever alone, making it crucial not to miss.
Sepsis: Bacterial sepsis can present subtly at first, with fever being an early sign, and is critical to identify promptly.
Kidney Infection (Pyelonephritis): Especially in young children, urinary tract infections can ascend to the kidneys, leading to pyelonephritis, which may initially present with nonspecific symptoms like fever.
Osteomyelitis or Septic Arthritis: Bone or joint infections can have a subtle onset, with fever being an early sign, and require prompt diagnosis to prevent long-term damage.

Kawasaki Disease: An acute febrile illness of childhood that, although rare, can lead to significant cardiac complications if not recognized and treated.
Juvenile Idiopathic Arthritis (JIA): Systemic JIA can present with fever of unknown origin, among other symptoms.
Leukemia or Lymphoma: These malignancies can occasionally present with prolonged fever as an initial symptom.
Tuberculosis: Especially in endemic areas or in children exposed to someone with TB, this infection can present with nonspecific symptoms including fever.