A fever is not an illness, but a sign the body is responding to an underlying issue, often an infection or inflammation. Normal body temperature ranges from 97.7°F to 99.5°F (36.5°C to 37.5°C). A high fever is a temperature at or above 103°F (39.4°C) for adults. For children, a fever is above 100.4°F (38°C). This elevated temperature is part of the body’s natural defense, creating an environment less favorable for pathogens.
The Body’s Internal Response
When a fever develops, the body’s internal thermostat in the hypothalamus raises its set point. This occurs due to pyrogens, substances from invading pathogens or the body’s immune cells. These pyrogens travel to the hypothalamus, resetting the temperature higher.
To reach this new, higher temperature, the body initiates physiological changes. Heat production increases through shivering, which generates heat via muscle activity. Heat loss is reduced by vasoconstriction, where blood vessels near the skin narrow, shunting blood away from the periphery to conserve warmth.
The elevated temperature impacts the immune system. A fever enhances immune function by stimulating white blood cells and increasing their activity. It promotes the production of acute phase proteins, important components of the immune response. The higher temperature also inhibits the growth of certain microbial agents, as many pathogens function optimally at normal body temperatures.
Recognizable Signs and Effects
As the body raises its internal temperature to the new set point, symptoms begin to manifest. Individuals experience chills and shivering, which are the body’s attempts to generate more heat. Once the fever peaks, the body cools down, leading to sweating and flushed, warm skin.
A high fever can cause other common effects. Headaches and muscle aches are frequently reported due to the body’s inflammatory response and increased metabolic activity. Fatigue and general weakness are also common, reflecting the energy demands placed on the body as it fights off infection. Many individuals may also experience a loss of appetite and increased thirst.
Particularly in children or the elderly, a high fever can lead to irritability or mild confusion. These symptoms are a direct result of the systemic changes occurring within the body. While uncomfortable, these signs indicate the body’s active engagement in fighting off the underlying cause of the fever.
Understanding Serious Complications
While most fevers are not inherently dangerous and resolve without severe issues, prolonged or extremely high temperatures can lead to serious complications. Dehydration is a concern, as increased sweating and metabolic rate lead to fluid loss. Adequate fluid intake is important to prevent this.
Febrile seizures can occur in young children, typically between 6 months and 5 years of age, though they are usually harmless and do not cause lasting effects. Delirium or significant confusion can also develop, especially with very high fevers, affecting mental clarity and awareness. Additionally, the increased metabolic demand during a fever can lead to a rapid heart rate (tachycardia), which may strain the cardiovascular system.
In rare and extreme instances, particularly with temperatures exceeding 105.8°F (41°C) or prolonged high fever, there is a risk of organ dysfunction or heatstroke. Such extreme temperatures can denature proteins and damage cells, impacting various organ systems. While these severe complications are uncommon, recognizing the potential risks underscores the importance of monitoring a high fever.
When to Consult a Doctor
Knowing when to seek medical attention for a fever is important. For infants younger than 3 months, any rectal temperature of 100.4°F (38°C) or higher warrants an immediate call to a doctor or a visit to the emergency room. For children aged 3 months or older, consult a doctor if the temperature is higher than 102.2°F (39°C), or if the fever persists for more than three days.
Adults should contact a healthcare provider if their temperature reaches 103°F (39.4°C) or higher, or if the fever does not respond to medication. Immediate medical attention is necessary for a fever accompanied by severe symptoms such as a stiff neck, severe headache, difficulty breathing, chest pain, a rash, persistent vomiting, or significant confusion. Individuals with chronic health conditions or weakened immune systems should also seek medical advice promptly for any fever.