A fever is a body temperature above 100.4°F (38°C). This threshold marks the point where the body's internal thermostat resets to a higher temperature in response to illness.
The hypothalamus, the brain's temperature regulator, triggers a fever when the immune system detects infection. This elevated temperature helps fight off pathogens by speeding up white blood cell production and slowing bacterial growth.
Fevers are classified by severity:
| Type | Temperature Range | Typical Causes |
|---|---|---|
| Low-grade fever | 100.4°F to 102.2°F (38°C to 39°C) | Viral infections, minor illnesses |
| Moderate fever | 102.2°F to 104°F (39°C to 40°C) | Bacterial infections, more serious illnesses |
| High fever | Above 104°F (40°C) | Severe infections, heat exhaustion |
While fevers are a normal immune response, seek medical help if:
According to the Mayo Clinic, prompt medical care is important for high-risk individuals, such as those with weakened immune systems or chronic illnesses.
Many people confuse a slight fever with normal body temperature fluctuations. The CDC notes that normal body temperature varies between 97°F (36.1°C) and 99°F (37.2°C). A slight fever starts at 100.4°F (38°C), not 99°F as many assume.
During flu season, my family member developed a temperature of 101.3°F (38.5°C). Recognizing this as a low-grade fever, we monitored symptoms and ensured hydration. The fever broke within 48 hours without complications, demonstrating how the body can fight mild infections independently.