Types of Fever
Fever can be classified by etiology (cause), temporal pattern, and clinical context, with the most clinically relevant classification being etiologic rather than pattern-based, as fever patterns have limited diagnostic utility in modern medicine.
Classification by Etiology
Tropical/Travel-Associated Fevers
When evaluating febrile patients with travel history, the etiologic breakdown is critical:
Tropical Infections (32.7% of febrile travelers): 1
- Parasitic infections dominate at 23.2% of all febrile cases, with malaria comprising 22.2% 1
- Viral infections account for 5.4%, predominantly dengue fever (5.2%) and chikungunya (0.2%) 1
- Bacterial infections represent 4.1%, including enteric fever (2.3%) and rickettsioses (1.7%) 1
Non-Tropical Infections (38.6% of febrile travelers): 1
- Acute diarrheal disease (13.6%) 1
- Respiratory tract infections (13.5%), including upper respiratory infections (6.0%), pneumonia (2.7%), and influenza (2.5%) 1
- Genitourinary tract infections (2.7%) 1
- Skin/soft tissue infections (2.5%) 1
Fever of Unknown Origin: 17.8% of cases remain undiagnosed despite workup 1
Classification by Underlying Disease Category
Infectious causes remain the most common, but fever can arise from: 2
- Infectious agents (bacterial, viral, parasitic, fungal)
- Neoplastic processes
- Immune or metabolic disorders
- Genetic abnormalities of inflammasome functioning 2
Classification by Temporal Pattern
While fever patterns have limited diagnostic value (with the exception of sustained fever in specific conditions), they are traditionally described as: 3
- Continuous fever: Persistent elevation with minimal variation (<1°C daily fluctuation)
- Remittent fever: Daily fluctuations >1°C but never reaching normal, typically following diurnal variation 3
- Intermittent fever: Temperature returns to normal between spikes 2, 4
- Hectic fever: Extreme fluctuations with temperature spikes and returns to normal or below 3
Critical caveat: A prospective study of 200 consecutive patients demonstrated that fever patterns (except sustained fever in gram-negative pneumonia or CNS damage) are not diagnostically helpful, as hectic fevers occurred across all disease categories and did not reliably predict bacteremia 3. Most patients had remittent or intermittent patterns regardless of underlying etiology 3.
Classification by Clinical Syndrome
Fever with Specific Associated Features
Fever with jaundice: 1
- Leptospirosis
- Viral hepatitis (A, B, E)
- Viral hemorrhagic fevers
- Yellow fever 1
Fever with hepatosplenomegaly: 1
- Amoebic liver abscess
- Brucellosis
- Trypanosomiasis
- Visceral leishmaniasis 1
Dengue-specific classifications: 5
- Dengue without warning signs: Acute febrile illness with frontal headache, retro-orbital pain, muscle/joint pain, and rash 5
- Dengue with warning signs: Includes persistent vomiting, abdominal pain/tenderness, clinical fluid accumulation, mucosal bleeding, lethargy/restlessness, hepatomegaly, hematocrit rise with thrombocytopenia 5
- Dengue hemorrhagic fever: Acute febrile illness with bleeding phenomena, thrombocytopenia, and plasma leakage 5
- Dengue shock syndrome: All DHF criteria plus hypotension or narrow pulse pressure (<20 mmHg) 5
Practical Clinical Approach
For returning travelers with fever, prioritize exclusion of malaria first (most important potentially fatal cause), then consider enteric fever if returning from Asia, as these represent the highest morbidity/mortality risks 1. The geographic region visited significantly influences differential diagnosis: Africa (51.1% of travelers), Asia (29.3%), and Latin America (11.3%) have distinct endemic disease patterns 1.
Normal body temperature ranges from 36.1°C to 37.2°C orally in healthy adults, with fever defined as temperature >37.5°C axillary or history of feeling hot/cold, chills, sweating, or headache 1, 6.