Fever, at four ages

14 days

Serious bacterial infection: bacteremia, meningitis, urinary tract infection; herpes simplex.

Differential

1. Late-onset group B streptococcal or E. coli sepsis
2. Bacterial meningitis
3. Urinary tract infection
4. Herpes simplex virus infection
5. Viral illness, a diagnosis of exclusion at this age

What changes

Full evaluation regardless of appearance: blood, urine and cerebrospinal fluid cultures, empiric antibiotics, admission. Consider herpes simplex if seizures, vesicles, raised liver enzymes or a maternal history.

6 months

Urinary tract infection and occult bacteremia if unimmunized; most fevers are viral.

Differential

1. Viral upper respiratory infection
2. Urinary tract infection (most common bacterial source)
3. Otitis media
4. Bronchiolitis or pneumonia
5. Bacteremia, now uncommon in the immunized

What changes

Appearance guides everything. Urinalysis and culture in a fever without a source; blood work if ill-appearing or unimmunized; observation with a clear return plan if well.

4 years

A source you can usually find on examination; the ill-appearing child with no source.

Differential

1. Viral pharyngitis or upper respiratory infection
2. Streptococcal pharyngitis
3. Otitis media
4. Pneumonia
5. Urinary tract infection (less common, especially in boys)

What changes

Examine for the source: throat, ears, chest, skin, joints. Investigations are targeted. A well child with a fever and a viral picture needs a return plan, not a workup. Fever beyond 5 days raises Kawasaki disease.

15 years

Adult-pattern infections plus adolescent-specific ones; the systemic illness that is not an infection.

Differential

1. Infectious mononucleosis
2. Streptococcal pharyngitis
3. Pneumonia
4. Sexually transmitted infection or pelvic inflammatory disease
5. Inflammatory or malignant disease presenting as fever

What changes

Take the history privately. Ask about sexual health, travel, drugs and weight loss. Examine the whole adolescent, including the lymph nodes and the spleen.