Introduction
A fever is a temporary rise in body temperature, usually as a response to infection, inflammation, or other medical conditions. While commonly associated with illness, not all temperature elevations indicate a serious problem. Understanding normal body temperature ranges by age and what constitutes a fever is essential for proper monitoring and timely medical care.
Body temperature can vary by age, time of day, activity level, and measurement method (oral, rectal, ear, forehead, or axillary).
Normal Body Temperature Ranges by Age
| Age Group | Normal Body Temperature |
| Newborns (0–3 months) | 97.9–100.4°F (36.6–38°C) |
| Infants (3–12 months) | 99–100.4°F (37.2–38°C) |
| Toddlers (1–3 years) | 98.6–100.4°F (37–38°C) |
| Children (4–10 years) | 98–100°F (36.7–37.8°C) |
| Older children and teens | 97.8–99.5°F (36.5–37.5°C) |
| Adults | 97–99°F (36.1–37.2°C) |
| Elderly | Slightly lower, often 96.8–98.3°F (36–36.8°C) |
Key points:
- Rectal temperatures are generally 0.5–1°F higher than oral measurements.
- Axillary (underarm) readings are usually 0.5–1°F lower than oral readings.
- Fever thresholds vary slightly by source but are generally accepted as above 100.4°F (38°C).
What Is Considered a Fever?
Infants and Children
- ≥100.4°F (38°C) generally indicates a fever
- Infants <3 Months with any fever should be evaluated by a healthcare provider promptly
- Fever in older children is often managed at home unless associated with persistent vomiting, rash, difficulty breathing, or lethargy
Adults
- Fever is generally ≥100.4°F (38°C)
- Mild fevers (100.4–102°F / 38–38.9°C) are often due to infections like colds or flu
- High fever (>104°F / 40°C) requires medical evaluation
Elderly
- Older adults may have a lower baseline temperature
- Fever may be milder even with serious infections; a temperature of 99°F (37.2°C) may indicate illness
Factors Affecting Body Temperature
- Time of day: Lowest in the morning, highest in late afternoon/evening
- Activity: Exercise increases body temperature temporarily
- Hormonal fluctuations: Menstrual cycle, thyroid disorders
- Medications: Some drugs (e.g., antibiotics, vaccines) may cause mild fever
When to Seek Medical Attention
- Infants <3 months with fever ≥100.4°F (38°C)
- Persistent fever lasting >3 days in children or adults
- High fever: ≥104°F (40°C)
- Associated symptoms like rash, breathing difficulty, confusion, seizure, severe pain, or dehydration
Conclusion
Fever is a common physiological response and varies by age, measurement method, and individual factors. While a mild fever is often self-limiting, knowing normal ranges by age and thresholds for concern ensures timely monitoring and appropriate medical care. Infants, elderly individuals, and anyone with a high or persistent fever require special attention to prevent complications.
FAQs
1. Is 100°F considered a fever?
In most children and adults, 100°F (37.8°C) is slightly elevated but not technically a fever; fever is generally ≥100.4°F (38°C).
2. Can body temperature vary throughout the day?
Yes, temperature is typically lowest in the morning and highest in the late afternoon/evening.
3. Are rectal temperatures more accurate than oral?
Yes, rectal readings are generally 0.5–1°F higher and considered more accurate in infants and young children.
4. Why might elderly people have lower fevers?
Aging may reduce the baseline body temperature and weaken the fever response to infection.
5. When should I call a doctor for a fever?
In infants <3 months, persistent high fever, or fever with serious symptoms such as difficulty breathing, seizures, confusion, or dehydration.
