Fever & Chills: Why Your Body Feels Cold When Hot

Introduction Fever and chills are frequently experienced together and can be quite uncomfortable. A fever is an elevation in your body's core temperature above its normal range, typically considered to be above 100.4°F (38°C). It is not an illness itself, but rather a sign that your body's immune system is actively working to combat an invader, such as bacteria or viruses, or responding to inflammation. This natural defense mechanism helps create an environment less hospitable for pathogens to thrive. Chills, on the other hand, are sensations of coldness accompanied by shivering, goosebumps, and paleness, even when your body temperature is rising. They often precede or accompany a fever. Your body induces chills as a way to generate heat and raise its core temperature to a new, higher set point dictated by the hypothalamus in your brain. This process of muscle contraction and shivering helps to increase heat production, contributing to the feeling of being cold despite an internal temperature increase. Common Causes Infections: Many types of infections, both viral and bacterial, can trigger fever and chills. Common examples include the flu, common cold, strep throat, bronchitis, pneumonia, and urinary tract infections (UTIs). Your immune system releases chemicals called pyrogens in response to these invaders, which signal the brain to raise body temperature. Influenza (Flu): A common viral respiratory illness, the flu frequently presents with sudden onset of fever, chills, body aches, headache, fatigue, and sometimes a cough or sore throat. Common Cold: While usually milder than the flu, a severe cold can sometimes cause a low-grade fever and chills, alongside symptoms like runny nose, sneezing, and sore throat. Urinary Tract Infections (UTIs): These bacterial infections can affect any part of the urinary system. When they become more severe or affect the kidneys, they may cause fever, chills, back pain, and painful urination. Gastroenteritis (Stomach Flu): Viral or bacterial infections of the digestive tract can lead to fever, chills, nausea, vomiting, diarrhea, and abdominal cramps. Tonsillitis or Strep Throat: Bacterial or viral infections of the tonsils or throat can cause a sore throat, difficulty swallowing, fever, and chills. Ear Infections: Especially common in children, middle ear infections (otitis media) can be accompanied by fever and chills, along with ear pain. Vaccinations: It is not uncommon to experience a low-grade fever and mild chills for a day or two after receiving certain vaccinations, as your immune system builds protection. This is a normal and expected response. Less Common Causes Autoimmune Diseases: Conditions like lupus, rheumatoid arthritis, or inflammatory bowel disease (IBD) can sometimes cause fever and chills as part of their inflammatory response. The immune system mistakenly attacks healthy body tissues, leading to systemic inflammation. Certain Medications: Some medications, particularly antibiotics or drugs used in chemotherapy, may list fever and chills as potential side effects. This is sometimes referred to as a drug fever. Blood Transfusion Reactions: Although rare with modern screening, an adverse reaction to a blood transfusion can sometimes manifest with fever, chills, and other symptoms shortly after the transfusion begins. Deep Vein Thrombosis (DVT): In some cases, a blood clot in a deep vein, typically in the leg, can cause inflammation that may lead to a low-grade fever and chills, alongside pain and swelling in the affected limb. Malignancy (Cancer): Some cancers, particularly lymphomas, leukemias, and kidney cancer, can cause unexplained fevers and chills as paraneoplastic syndromes or due to the body's response to the cancer cells. Associated Symptoms Fever and chills rarely occur in isolation and are often accompanied by a range of other symptoms that can help healthcare providers narrow down the potential cause. These may include general malaise, a feeling of being unwell,