Fever

A fever is when your body gets warm.

It helps you fight germs. Your brain acts like a heater. It tells your body to make more heat.

This can make you feel cold. You might shake or have chills. This helps your body stay warm.

When the fever goes away, you feel hot. You might sweat to cool down.

Fevers can happen for many reasons. They can come from tiny germs.

Some people feel very sleepy. You might even have strange dreams.

It is a way your body works to keep you safe.

A fever is when your body temperature goes up. It is a way your body fights germs.

Your brain has a part called the hypothalamus. It works like a thermostat. This part sets your body's target temperature.

Sometimes, a substance called a pyrogen tells the brain to raise that target. This happens during infections from viruses or bacteria. To reach this new, higher target, your body makes more heat. You might shake or have chills. This is called shivering. Shivering uses muscle movements to make heat. Your body also pulls blood away from your skin. This helps keep the heat inside.

When the target temperature goes back to normal, you may feel hot. You might sweat or look flushed. This is how your body cools down. Fevers can cause other things, too. You might feel sleepy or lose your appetite. Some people even have strange dreams.

A fever is different from hyperthermia. Hyperthermia happens when the body gets too hot from the outside or cannot lose heat. Fever is a helpful defense.

A fever is a common sign that your body is working to protect itself. It is a defense mechanism used to fight infections.

When you have a fever, your body temperature rises above its usual range. This happens because a part of your brain called the hypothalamus changes its target temperature. You can think of the hypothalamus as a thermostat for your body. Normally, it keeps your temperature at a steady level. During a fever, the thermostat is turned up to a higher setting. This helps your body deal with germs like viruses or bacteria.

To reach this new, higher temperature, your body goes through several steps. First, a substance called a pyrogen triggers the process. These pyrogens can come from outside the body, like from bacteria, or from inside your own immune system. The pyrogens cause the release of a chemical called prostaglandin E2, or PGE2. This chemical tells the hypothalamus to raise the set point. To make up the heat, your body might start shivering. Shivering is when your muscles move quickly to create warmth. Your body also uses vasoconstriction, which means it pulls blood away from your skin to keep heat inside.

Many different things can cause a fever to start. It is often caused by infections like the common cold, influenza, or COVID-19. Other infections like malaria or meningitis can also cause it. Sometimes, things that are not infections cause fevers too. These include things like certain medications, vaccinations, or even cancer. It is important to know that fever is different from hyperthermia. In hyperthermia, the body gets too hot because of the environment or because it cannot lose heat. In hyperthermia, the brain's thermostat stays at the normal setting. This is a dangerous situation that needs quick treatment.

Doctors use different ways to check your temperature. They might check your mouth, your ear, or your forehead. Some use a rectal thermometer, which is often the most accurate way to find a central temperature. In healthy adults, a normal temperature is often around 37°C (98.6°F) when measured in the rectum. However, normal temperatures can change based on your age or the time of day. A fever is generally considered to be present if the temperature is at or over 38°C (100.4°F) in the rectum.

Doctors must be careful with very young babies or people who are already quite sick.

Having a fever can make you feel quite different. You might feel very sleepy, lose your appetite, or feel sad. Some people even experience "fever dreams," which are strange or confusing nightmares. You might also feel very thirsty or have a hard time concentrating. While fevers can be uncomfortable, they do not always need medicine to go away. Medicines like ibuprofen or paracetamol can help reduce pain or lower the temperature if needed. Most of the time, a fever is just your body's way of staying strong against germs.

Fever, or pyrexia, is a common medical sign. It is an anti-infection defense mechanism in humans. A fever occurs when the body temperature exceeds the normal range. This happens because of an increase in the body's temperature set point. This set point is controlled by the hypothalamus in the brain. You can think of the hypothalamus as a biological thermostat.

When the thermostat is turned up, the body works to reach that new level.

The mechanism of fever is a complex chain of events. It begins when a substance called a pyrogen triggers a response. Pyrogens can be exogenous, meaning they come from outside the body. Examples include bacterial lipopolysaccharides (LPS) from Gram-negative bacteria like E. coli. Pyrogens can also be endogenous, meaning they are produced inside the body. These include cytokines like interleukin 1 (IL-1), interleukin 6 (IL-6), and tumor necrosis factor-alpha (TNFα). These chemicals are released by immune cells like monocytes and macrophages.

Once released, these pyrogens lead to a specific chemical pathway. They migrate to the brain's circumventricular organs to bypass the blood-brain barrier. This process activates the arachidonic acid pathway. This pathway uses enzymes like phospholipase A2 (PLA2) and cyclooxygenase-2 (COX-2). These enzymes facilitate the synthesis of prostaglandin E2 (PGE2). PGE2 is the ultimate mediator of the febrile response. It acts on EP3-expressing neurons in the preoptic area (POA) of the hypothalamus. This action inhibits certain neurons, which then activates the body's heat-generating systems.

To reach the new, higher set point, the body uses several heat-generating methods. One method is shivering, which is rapid muscle contraction to produce heat. The body also uses norepinephrine to increase thermogenesis in brown adipose tissue. In infants, this non-shivering thermogenesis is very important. Another method is peripheral vasoconstriction. This process reduces heat loss through the skin by narrowing blood vessels. This often makes a person feel cold or experience chills.

When the set point returns to normal, the body must cool down. This can happen spontaneously or through the use of medication. The body uses processes like sweating and vasodilation to release heat. Vasodilation is the widening of blood vessels to increase heat loss. A person might feel hot and become flushed during this stage. Fever is distinct from hyperthermia, which is a different medical condition. In hyperthermia, the temperature rises above the set point because of too much heat production or poor heat loss. Hyperthermia does not respond to antipyretic medications and can be life-threatening.

Fever is associated with many different medical conditions. It can be caused by viral, bacterial, or parasitic infections. Examples include influenza, COVID-19, malaria, and meningitis. Non-infectious causes also exist, such as cancer or certain vaccinations. Fever is a very common symptom in clinical settings. It is part of about 30% of healthcare visits for children. In adults who are seriously ill, fever occurs in up to 75% of cases.

While uncomfortable, treating a fever does not appear to change medical outcomes.

A fever often brings other symptoms, known as sickness behavior. These include lethargy, sleepiness, and a loss of appetite. Some people experience hyperalgesia, which is an increased sensitivity to pain. High fevers can also cause delirium or hallucinations. Some people experience intense, confusing nightmares called "fever dreams." In rare cases, a fever may trigger a febrile seizure, which is more common in young children. Doctors monitor temperature using different methods. Rectal temperatures are considered more accurate for central temperature. A fever is generally defined as a rectal temperature of 38°C (100.4°F) or higher.