Although 98.6° F (37° C) is considered normal temperature, body temperature varies throughout the day. It is lowest in the early morning and highest in the late afternoon—sometimes reaching 99.9° F (37.7° C). Similarly, a fever does not stay at a constant temperature. Sometimes temperature peaks every day and then returns to normal. This process is called intermittent fever. Alternatively, temperature varies but does not return to normal. This process is called remittent fever. Doctors no longer think that the pattern of the rise and fall of fever is very important in the diagnosis of certain disorders. Show
Body temperature can be measured at several areas of the body. The most common sites are the mouth (oral) and rectum. Other sites include the ear, forehead, and, much less preferably, armpit. Temperature is typically measured using a digital thermometer. Glass thermometers containing mercury are no longer recommended because they can break and expose people to mercury. Oral temperatures are considered elevated when
Rectal and ear temperatures are about 1.0° F (0.6° C) higher than oral temperatures. Skin temperatures (for example, the forehead) are about 1.0° F (0.6° C) lower than oral temperatures. Many people use the term “fever” loosely, often meaning that they feel too warm, too cold, or sweaty, but they have not actually measured their temperature. The ability to generate a fever is reduced in certain people (for example, those who are very old, very young, or who have an alcohol use disorder). The symptoms people have are due mainly to the condition causing the fever rather than the fever itself. Although many people worry that fever can cause harm, the typical temporary elevations in body temperature ranging from 100.4° to 104° F (38° to 40° C) caused by most short-lived (acute) infections are well-tolerated by healthy adults. However, a moderate fever may be slightly dangerous for adults with a heart or lung disorder because fever causes the heart rate and breathing rate to increase. Fever can also worsen mental status in people with dementia. Substances that cause fever are called pyrogens. Pyrogens can come from inside or outside the body. Microorganisms and the substances they produce (such as toxins) are examples of pyrogens formed outside the body. Pyrogens formed inside the body are usually produced by monocytes and macrophages (two types of white blood cells). Pyrogens from outside the body can cause fever by stimulating the body to release its own pyrogens or by directly affecting the area of the brain that controls temperature Fever . Infection is not the only cause of fever. Fever may also result from inflammation, a reaction to a drug, an
allergic reaction
Overview of Allergic Reactions Allergic reactions (hypersensitivity reactions) are inappropriate responses of the immune system to a normally harmless substance. Usually, allergies make people sneeze; the eyes water and itch... read more
Many disorders can cause fever. They are broadly categorized as
An infectious cause is highly likely in adults with a fever that lasts 4 days or less. A noninfectious cause is more likely to cause fever that lasts a long time or returns. Many cancers cause fever. Inflammatory disorders
that cause fever include joint, connective tissue, and blood vessel disorders such as rheumatoid arthritis
Rheumatoid Arthritis (RA) Rheumatoid arthritis is an inflammatory arthritis in which joints, usually including those of the hands and feet, are inflamed, resulting in swelling, pain, and often destruction of joints.... read more
Also, an isolated, short-lived (acute) fever in people with cancer or a known inflammatory disorder is most likely to have an infectious cause. In healthy people, an acute fever is unlikely to be the first sign of a chronic illness. Drugs sometimes cause fever. For example, beta-lactam antibiotics (such as penicillin Penicillins Penicillins are a subclass of antibiotics called beta-lactam antibiotics (antibiotics that have a chemical structure called a beta-lactam ring). Carbapenems, cephalosporins, and monobactams... read more ) and sulfa drugs can trigger a fever. Drugs that can cause an extremely high temperature include certain illicit drugs (such as cocaine Cocaine Cocaine is an addictive stimulant drug made from leaves of the coca plant. Cocaine is a strong stimulant that increases alertness, causes euphoria, and makes people feel powerful. High doses... read more , amphetamines Amphetamines Amphetamines are stimulant drugs that are used to treat certain medical conditions, but are also subject to abuse. Amphetamines increase alertness, enhance physical performance, and produce... read more , or phencyclidine Ketamine and Phencyclidine (PCP) Ketamine and phencyclidine are chemically similar drugs used for anesthesia but are sometimes used recreationally. Ketamine is available in powder and liquid form. The powder can be snorted... read more ), anesthetics Anesthetics Pain relievers (analgesics) are the main drugs used to treat pain. Doctors choose a pain reliever based on the type and duration of pain and on the drug's likely benefits and risks. Most pain... read more , and antipsychotic drugs Antipsychotic drugs Schizophrenia is a mental disorder characterized by loss of contact with reality (psychosis), hallucinations (usually, hearing voices), firmly held false beliefs (delusions), abnormal thinking... read more . Virtually all infectious disorders can cause fever. But overall, the most likely infectious causes are
Most acute respiratory tract and gastrointestinal infections are viral. Certain conditions (risk factors) make people more likely to have a fever. These factors include the following:
 Usually, a doctor can determine that an infection is present based on a brief history, a physical examination, and occasionally a few simple tests, such as a chest x-ray and urine tests. However, sometimes the cause of fever is not readily identified. When doctors initially evaluate people with an acute fever, they focus on two general issues:
In people with an acute fever, certain signs and characteristics are cause for concern. They include
People who have any warning signs should see a doctor right away. Such people typically need immediate testing and often admission to a hospital. People without warning signs should call the doctor if the fever lasts more than 24 to 48 hours. Depending on the person's age, other symptoms, and known medical conditions, the doctor may ask the person to come for evaluation or recommend treatment at home. Typically, people should see a doctor if a fever lasts more than 3 or 4 days regardless of other symptoms. Doctors first ask questions about the person's symptoms and medical history. Doctors then do a physical examination. What they find during the history and physical examination often suggests a cause of the fever and the tests that may need to be done. A doctor begins by asking a person about present and previous symptoms and disorders, drugs currently being taken, any blood transfusions, exposure to infections, recent travel, vaccinations, and recent hospitalizations, surgeries, or other medical procedures. The pattern of the fever rarely helps the doctor make a diagnosis. However, a fever that returns every other day or every third day is typical of malaria. Doctors consider malaria as a possible cause only if people have traveled to an area where malaria is common. Recent travel may give the doctor clues to the cause of a fever because some infections occur only in certain areas. For example, coccidioidomycosis (a fungal infection) occurs almost exclusively in the southwestern United States. Recent exposures are also important. For example, people who work in a meatpacking plant are more likely to develop brucellosis (a bacterial infection spread through contact with domestic animals). Other examples include unsafe water or food (such as unpasteurized milk and milk products, and raw or undercooked meat, fish, and shellfish), insect (such as mosquitoes) and tick bites, unprotected sex, and occupational or recreational exposures (such as hunting, hiking, and water sports). Pain is an important clue to the possible source of fever, so the doctor asks about any pain in the ears, head, neck, teeth, throat, chest, abdomen, flank, rectum, muscles, and joints. Other symptoms that help determine the cause of the fever include nasal congestion and/or discharge, cough, diarrhea, and urinary symptoms (frequency, urgency, and pain while urinating). Knowing whether the person has enlarged lymph nodes or a rash (including what it looks like, where it is, and when it appeared in relation to other symptoms) may help the doctor pinpoint a cause. People with recurring fevers, night sweats, and weight loss may have a chronic infection such as tuberculosis or endocarditis (infection of the heart's lining and usually the heart valves). The doctor may also ask about the following:
The physical examination begins with confirmation of fever. Fever is most accurately determined by measuring rectal temperature, but oral or ear temperature is often measured. Forehead temperature is not as accurate as rectal, but because of the COVID-19 pandemic, doctors may measure forehead temperature to screen people for fever. Armpit temperature is the least accurate, so doctors rarely use it to confirm a fever. Doctors then do a thorough examination from head to toe to check for a source of infection or evidence of disease. The need for testing depends on what the doctor finds during the medical history and physical examination. Otherwise healthy people who have an acute fever and only vague, general symptoms (for example, they feel generally ill or achy) probably have a viral illness that will go away without treatment. Therefore, they do not usually require testing. Exceptions are people in an area where COVID-19 is being spread, people who have been exposed to an animal or insect that carries and transmits a specific disease (called a vector), such as people with a tick bite, and people who have recently been in an area where a particular disorder (such as malaria) is common. If otherwise healthy people have findings that suggest a particular disorder, testing may be needed. Doctors select tests based on those findings. For example, if people have a headache and stiff neck, a spinal tap (lumbar puncture) is done to look for meningitis. If people have a cough and lung congestion, a chest x-ray is done to look for pneumonia. For certain respiratory diseases, such as COVID-19 or influenza, there are rapid molecular tests that can give results within minutes or a few hours. Some rapid tests can be done at home or in a doctor's office on a sample obtained with a cotton swab. Other rapid tests to identify a specific cause of an infection require a blood sample sent to a laboratory. People who are at increased risk of infection, people who appear seriously ill, and older people often need testing even when findings do not suggest a particular disorder. For such people, doctors may do the following:
An increase in the white blood cell count usually indicates infection. The proportion of different types of white blood cells (differential count) gives further clues. For example, an increase in neutrophils suggests a relatively new bacterial infection. An increase in eosinophils suggests the presence of parasites, such as tapeworms or roundworms. Also, blood, urine, and other body fluids may be sent to the laboratory to try to grow the microorganism in a culture. Other blood tests can be used to detect antibodies against specific microorganisms. A fever of unknown origin may be diagnosed when
In such cases, the cause may be an unusual chronic infection (such as
tuberculosis Tuberculosis (TB)
Tuberculosis is a chronic contagious infection caused by the airborne bacteria Mycobacterium tuberculosis. It usually affects the lungs, but almost any organ can be involved. Tuberculosis... read more
Ultrasonography Ultrasonography Ultrasonography
uses high-frequency sound (ultrasound) waves to produce images of internal organs and other tissues. A device called a transducer converts electrical current into sound waves... read more If these test results are negative, doctors may need to take a sample of tissue from the liver, bone marrow, or another site of suspected infection for biopsy. The sample is then examined under a microscope, cultured, and analyzed. The best fever treatment is treatment of the cause. Because fever helps the body defend against infection and because fever itself is not dangerous (unless it is higher than about 106° F [41° C]), there is some debate as to whether fever should be routinely treated. However, people with a high fever generally feel much better when the fever is treated. Plus, people with a heart or lung disorder and those with dementia are considered to be at particular risk of dangerous complications, so when they have a fever, it should be treated. Drugs used to lower body temperature are called antipyretics. The most effective and widely used antipyretics are acetaminophen and nonsteroidal anti-inflammatory drugs (NSAIDs), such as aspirin, ibuprofen, and naproxen. These drugs are taken according to the directions given on the container's label. Because many over-the-counter cold or flu preparations contain acetaminophen, people must be careful not to take acetaminophen and one or more of these preparations at the same time. Other cooling measures (such as cooling with a tepid water mist and using cooling blankets) are needed only if the temperature is about 106° F (41.1° C) or higher. Sponging with alcohol is avoided because alcohol can be absorbed through the skin and may have harmful effects. People who have a blood infection or who have abnormal vital signs (such as low blood pressure and a rapid pulse and breathing rate) are admitted to the hospital. Fever can be tricky in older people because the body may not respond the way it would in younger people. For example, in frail older people, infection is less likely to cause fever. Even when elevated by infection, the temperature may be lower than the standard definition of fever, and the degree of fever may not correspond to the severity of the illness. Similarly, other symptoms, such as pain, may be less noticeable. Frequently, a change in mental function or a decline in daily functioning is the only other initial sign of pneumonia Hospital-Acquired Pneumonia Hospital-acquired pneumonia is lung infection that develops in people who have been hospitalized, typically after about 2 days or more of hospitalization. Many bacteria, viruses, and even fungi... read more or a urinary tract infection Overview of Urinary Tract Infections (UTIs) In healthy people, urine in the bladder is sterile—no bacteria or other infectious organisms are present. The tube that carries urine from the bladder out of the body (urethra) contains no bacteria... read more .
However, older people with a fever are more likely to have a serious bacterial infection than are younger adults with a fever. As in younger adults, the cause is commonly a respiratory or urinary tract infection. Skin and soft-tissue infections are also common causes in older people. Older people are more likely to have serious manifestations of respiratory viral infections such as influenza Influenza (Flu) Influenza (flu) is a viral infection of the lungs and airways with one of the influenza viruses. It causes a fever, runny nose, sore throat, cough, headache, muscle aches (myalgias), and a general... read more , COVID-19 COVID-19 COVID-19 is an acute respiratory illness that can be severe and is caused by the coronavirus named SARS-CoV-2. Symptoms of COVID-19 vary significantly. Two types of tests can be used to diagnose... read more , and respiratory syncytial virus Respiratory Syncytial Virus (RSV) Infection and Human Metapneumovirus Infection Respiratory syncytial virus infection and human metapneumovirus infection cause upper and sometimes lower respiratory tract infections. Respiratory syncytial virus is a very common cause of... read more (RSV).
Which occurs when the body temperature is lower than the normal average of 37 degrees Celsius?Hypothermia is a medical emergency that occurs when your body loses heat faster than it can produce heat, causing a dangerously low body temperature. Normal body temperature is around 98.6 F (37 C). Hypothermia (hi-poe-THUR-me-uh) occurs as your body temperature falls below 95 F (35 C).
What happens if the body temperature rises above 37?When the body core temperature rises above the critical level of 37 C (98.6 F), there is an increase in the rate of heat loss by sweating. Shivering and chemical thermogenesis are strongly inhibited.
What happens when the body temperature rises up significantly beyond the normal temperature which is 37 degrees Celsius a dehydration?Heat stroke can occur when your body temperature reaches above 104°F (40°C).
Why is the human body temperature 37 degrees Celsius?Scientists have found the reason why our body temperature is 37°C. Apparently it's the perfect balance, as it's warm enough to prevent fungal infection but not so hot that we need to eat nonstop to maintain our metabolism. Scientists have always wondered why advanced mammals are so hot compared to other animals.
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