Fever above 39°C: viral or bacterial?

Jul 15, 2025 Source: Cainiu Health

Dr. Ren Yi

Introduction

Fever above 39°C may indicate either a viral or bacterial infection. If accompanied by symptoms such as generalized muscle pain, fatigue, dry cough, and sore throat, and blood tests show normal or decreased white blood cell count with increased lymphocyte percentage, it is more likely a viral infection. Such cases are commonly seen in influenza, novel coronavirus infection, and others. After viral invasion, a systemic inflammatory response is triggered, leading to high fever.

　　Fever, also known as pyrexia, with a body temperature above 39°C may be caused by either a viral infection or a bacterial infection. It cannot be determined solely by body temperature and needs to be differentiated by combining other symptoms and medical examinations. If abnormalities occur, timely medical consultation is recommended. Detailed analysis is as follows:

　　If accompanied by symptoms such as generalized muscle aches, fatigue, dry cough, and sore throat, and routine blood tests show normal or decreased white blood cell count with an increased proportion of lymphocytes, it is mostly a viral infection. This situation is commonly seen in infections such as influenza and coronavirus. After the virus invades the body, it triggers a systemic inflammatory response leading to high fever.

　　If accompanied by symptoms such as purulent discharge, severe cough, chest pain, frequent urination, urgency, and painful urination, and routine blood tests show elevated white blood cell count and increased proportion of neutrophils, it is mostly a bacterial infection. This situation is commonly seen in conditions such as pneumonia, bacterial tonsillitis, and urinary tract infections. Bacterial proliferation stimulates a strong immune response from the body, causing high fever.

　　When a high fever exceeding 39°C occurs, timely medical consultation and examination are necessary to determine the type of infection before initiating targeted treatment, avoiding self-medication that may delay the progression of the illness.

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Is a high white blood cell count caused by a virus or bacteria?

Under normal circumstances, whether an elevated white blood cell count is caused by a virus or bacteria needs to be determined based on specific conditions, as both viral and bacterial infections can lead to increased white blood cells. In the early stages of viral infection, white blood cell counts may also rise, such as in poliomyelitis, measles, and varicella-zoster virus infection. An elevated white blood cell count during viral infection is a normal immune response that helps clear the virus.