Definition
The term fever of unknown origin (FUO) is best reserved for children with a fever documented by a health care provider and for which the cause could not be identified after 3 wk of evaluation as an outpatient or after 1 wk of evaluation in hospital.
Nelson Text Book 18thEdition
Ergönül O, Willke A, Azap A, et al. Revised definition of ‘fever of …

Exanthematous Illness

Exanthems – eruption of skin accompanied by inflammation.
Enanthems – eruption of mucous membrane.

Maculopapular rashes :

Measles
Rubella
Erythema Infectiosum
Roseola
Coxsackie
Echo virus
CMV and HBV
Erythema Multiforme due to Herpes virus, EBV, Adeno virus, Chlamydia, Salmonella, Mycobacteria, Histoplasma, Coccidiosis

Nodular rashes :

Fungal disease
Atypical Mycobacteria
Pseudomonas infection
Erythema Nodosum – Streptococcus, Mycobacteria, Yersinia, HCV, Sarcoidosis, Drugs and IBD

Diffuse erythematous with peeling or desquamation :

Scarlet fever
Toxic shock …