Almost any infection can cause a fever, making it one of the most common symptoms in medicine. Viral infections like the flu and common cold are the most frequent culprits, but bacterial infections, autoimmune diseases, certain cancers, and even some medications can raise your body temperature above the standard fever threshold of 100.4°F (38.0°C). Fever itself isn’t a disease. It’s your body’s defense response to something it perceives as a threat.
How Your Body Creates a Fever
When your immune system detects an invader, like a virus or bacteria, specialized cells called macrophages release signaling chemicals (cytokines) into your bloodstream. These signals travel to a temperature-control center deep in your brain, which functions like a thermostat. Normally, this thermostat keeps your body at roughly 98.6°F. But when those immune signals arrive, they trigger the production of a compound called prostaglandin E2, which literally resets the thermostat to a higher number.
Once the set point rises, your body treats its current normal temperature as “too cold.” That’s why you shiver and feel chilled at the start of a fever: your muscles are generating heat to reach the new, higher target. Some bacterial toxins can bypass the usual signaling chain entirely and act directly on the brain’s thermostat, which is one reason certain bacterial infections can spike fevers rapidly.
Viral Infections
Viruses are the single most common cause of fever. Well over 100 viruses can infect the respiratory tract alone, including rhinoviruses (the common cold), influenza A and B, coronaviruses, parainfluenza, adenoviruses, and respiratory syncytial virus (RSV). Fever from a typical respiratory virus lasts about 3 to 5 days.
Influenza tends to hit harder than most. High fevers above 103.1°F (39.5°C) occur in more than 50% of children with influenza A. The illness is usually self-limited, and antiviral treatment, when started early, can shorten symptoms by about a day and a half.
Mononucleosis, caused by the Epstein-Barr virus, is a standout because fever appears in virtually 100% of cases and can persist far longer than a typical viral illness. Mono fevers last an average of two weeks, though they can range from 4 days to 3 weeks, usually peaking around the fifth day. The temperature typically fluctuates between 101.3°F and 103.1°F in an on-and-off pattern rather than staying constantly elevated.
COVID-19, stomach viruses (norovirus, rotavirus), measles, and chickenpox are other well-known viral causes. In most cases, the fever resolves on its own as your immune system clears the virus.
Bacterial Infections
Bacterial infections often produce higher or more persistent fevers than viruses, though there’s significant overlap. Some of the most common bacterial causes include:
- Urinary tract infections (UTIs): Lower UTIs may cause a mild fever, but a kidney infection (pyelonephritis) typically produces a high fever with flank pain and chills.
- Strep throat: A sudden sore throat with fever above 101°F, no cough, and swollen lymph nodes is the classic pattern.
- Bacterial pneumonia: Tends to cause sustained high fevers with productive cough and chest pain, often feeling more severe than viral pneumonia.
- Skin infections: Cellulitis and abscesses can produce fever, especially when the infection spreads beyond a small area.
- Ear infections: Particularly common in young children, often with fever and ear pain.
One key difference: bacterial infections generally require antibiotics to resolve, while viral fevers do not. A fever that returns after initially improving, or one that worsens after 3 to 5 days, can sometimes signal a secondary bacterial infection layered on top of a virus.
Autoimmune and Inflammatory Diseases
Not every fever comes from an infection. Your immune system can generate the same inflammatory signals without any virus or bacteria present, which is exactly what happens in autoimmune conditions. Systemic lupus erythematosus (lupus), rheumatoid arthritis, adult-onset Still’s disease, sarcoidosis, polymyalgia rheumatica, and giant cell arteritis can all produce recurring or persistent low-grade fevers.
These fevers often follow a different pattern than infectious ones. They tend to be lower grade, come and go over weeks or months, and occur alongside other symptoms like joint pain, fatigue, rashes, or unexplained weight loss. When a fever lasts more than three weeks without an obvious infectious cause, doctors begin considering these inflammatory conditions as part of a workup for “fever of unknown origin.”
Cancers That Cause Fever
Certain cancers produce fevers, sometimes as one of the earliest noticeable symptoms. Lymphoma and leukemia are the most strongly associated with unexplained fevers, but liver cancer, colon cancer, pancreatic cancer, kidney cancer, and metastatic disease can all trigger them. These fevers occur because tumor cells release inflammatory substances, or because the cancer disrupts normal immune function.
Cancer-related fevers often appear as persistent low-grade temperatures, sometimes with drenching night sweats or unintentional weight loss. A fever that lingers for weeks without any infection and doesn’t respond to typical treatments warrants thorough investigation.
Medications and Vaccines
Drug fever is a real and underrecognized phenomenon. Several common medications can trigger a fever as a side effect, including certain blood pressure medications, blood thinners, seizure medications, gout treatments, diuretics, barbiturates, and antihistamines. Drug fevers can start days to weeks after beginning a new medication, making them easy to miss. The fever typically resolves once the medication is stopped.
Vaccines also commonly cause short-lived fevers, and this is a normal immune response rather than a sign of illness. Post-vaccination fevers are usually low grade and resolve within 2 to 3 days. They begin soon after the shot and reflect your immune system learning to recognize the target pathogen.
Heatstroke Is Not a Fever
It’s worth understanding the difference between a fever and hyperthermia from heat exposure. In a fever, your brain deliberately raises your body’s temperature set point as a defense mechanism. In heatstroke, your body’s cooling system fails and temperature rises uncontrollably, potentially reaching 106°F or higher within 10 to 15 minutes. Heatstroke is a medical emergency. Standard fever reducers won’t work because the problem isn’t an elevated set point; it’s that the body physically cannot cool itself down.
Fever Thresholds by Age
A temperature of 100.4°F (38.0°C) or higher is the widely accepted definition of fever for all ages. Beyond that baseline, what counts as concerning depends heavily on how old you are.
For adults, a “high fever” generally starts at 103.1°F (39.5°C). Most adult fevers from common infections are uncomfortable but not dangerous. For children between 3 months and 3 years, 102.2°F (39.0°C) measured rectally is considered the threshold for high fever. Children under 3 months are in a category of their own: any temperature of 100.4°F or higher in an infant younger than 12 weeks requires immediate medical evaluation, regardless of how the baby looks or acts.
Warning Signs That Need Urgent Attention
Most fevers, especially from common viral infections, resolve safely on their own. But certain symptoms alongside a fever point to potentially dangerous conditions. A stiff neck, where you or your child cannot touch chin to chest, can be an early sign of meningitis. Sudden confusion, seeing things that aren’t there, or not recognizing familiar people signals something affecting the brain that needs emergency care.
Purple or blood-red spots on the skin that don’t fade when you press on them, combined with fever, can indicate a serious bloodstream infection. In young children, you can test for a stiff neck by laying the child down and gently lifting their head toward their chest; if they resist strongly, that’s meaningful. Any of these symptoms paired with a fever warrants immediate emergency evaluation.