Sudden Fever – Causes of Acute Fever in Adults

The onset of a fever may at times be the first indication of an illness. It is difficult to to ascertain the exact cause of a fever without the presence of other signs and symptoms and in acute cases, a fever is most likely due to an infection. In a healthy adult, a mild fever may be transient and the body is able to cope without any risk to the health of a person. However, high fevers (above 40 degrees Celsius/104 degrees Farenheit) or a persistent fever may be an indication of more serious underlying causes that need to be isolated and treated as soon as possible.

A sudden fever is one of quick onset in a patient who was otherwise feeling well. It is most likely due to an infection with the most common sites being affected in an adult being the :

  • Upper respiratory tract
  • Gastrointestinal tract
  • Urinary tract
  • Skin

Typically localized symptoms are evident – pain, changes in normal functioning, discharge or swelling. However, the lack of signs and symptoms should not mean that the fever can be ignored, especially if the body temperature continues to rise or if the fever is persisting. The signs and symptoms of more serious causes, like sepsis, can beĀ  vague except for the presence of a high fever and is life-threatening without prompt medical attention.

Infectious Causes

Bacteria and viruses are the most common infectious causes of a fever. Fungal and protozoal infections are less common causes but should not be ignored, especially for travelers who recently visited endemic areas.

Upper Respiratory Tract

Common conditions : acute sinusitis, pharyngitis (strep throat, tonsillitis).

  • Discharge – runny nose, nasal congestion (sneezing), phlegm (coughing). The presence of pus may be indicative of a bacterial infection.
  • Pain – sore throat, burning nose/eyes
  • Changes in voice (hoarseness, nasal tone)
  • Diminished sense of taste and/or smell

Lower Respiratory Tract

Common conditions : bronchitis, pneumonia, lung abscess, empyema.

Gastrointestinal Tract

Common conditions : gastroenteritis – bacterial and viral.

  • Nausea
  • Vomiting
  • Diarrhea
  • Abdominal pain

Urinary Tract

Common conditions : bacterial urinary tract infections (UTIs).

  • Painful urination
  • Blood in the urine
  • Urethral discharge
  • Frequent urination
  • Urinary incontinence


Common conditions : abscess, cellulitis, insect bites/stings.

  • Itching, pain
  • Redness
  • Warmth
  • Swelling
  • Discharge (sometimes)

Other Causes

Associated Symptoms

The presence of the following symptoms may be useful for identifying the cause of the fever :

  • Headache
    • Meningitis
    • Encephalitis
    • Brain abscess
    • Endopthalmitis – pain behind the eye
    • Otitis media – ear pain
  • Abdominal Pain
    • Appendicitis
    • Hepatitis
    • Abscess (liver, pancreas, ovarian)
    • Infective cholecystitis (galbladder)
    • Pyelonephritis (kidney)
    • Colitis
    • Diverticulitis.
  • Chest Pain
    • Tuberculosis
    • Empyema
    • Lung abscess
    • Pericarditis
    • Pulmonary embolism
    • Rheumatic fever
  • Limb Pain (Upper ~ Arm, Lower ~ Leg)
    • Impetigo
    • Erysipelas
    • Cellulitis
    • Necrotizing fascitis
    • Deep vein thrombosis (DVT)


Patients who have been recently hospitalized or visited a health care facility should always be aware of nosocomial infections (hospital-acquired infections), post-operative infections, drugs and reactions to transfusions as a possible cause of a sudden fever. Other risks in hospital include IV catheters and ventilators (pneumonia).

Animals and Insects

Vector exposure should always be considered even if no visible bites/stings are evident. Many animals and insects are carriers of a number of infections.

  • Ticks
  • Fleas
  • Mosquitoes
  • Animals
  • Domestic – dogs, cats, rodents, birds
  • Wild – birds, bats, rodents
  • Reptiles


Refer to the article on Drug Fever for a complete list of substances responsible for drug-induced fever and drug-induced hyperthermia.

  • Narcotics
    • Cocaine
    • Ecstasy
    • Amphetamines
  • Pharmaceutical
    • Anesthetics
    • Antipsychotics
  • Allergic reactions to certain drugs :
    • Antibiotics, antivirals and antifungals
    • Anti-epileptics
    • Anti-arrhythmic drugs
    • Chemotherapy

Other Causes of an Elevated Body Temperature

  • Excessive alcohol consumption.
  • Heat stroke.
  • Recent vaccinations

Less common causes

There are various other causes of a fever in adults, some of which may not cause a fever of sudden onset. These conditions are usually chronic in nature and include autoimmune diseases, malignant tumors and chronic infections in immunocompromised patients (like in HIV/AIDS).

Related Articles

  1. What is a Fever (Pyrexia)? Normal and High Body Temperature

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