Viral upper respiratory tract infection (e.g., common cold, influenza): This is the most common cause of a short-term fever, often accompanied by other symptoms such as cough, sore throat, and runny nose.

Bacterial pharyngitis (e.g., strep throat): A common cause of fever, especially in children and young adults, often presenting with a sore throat and swollen lymph nodes.
Influenza: A highly contagious respiratory illness that can cause fever, cough, sore throat, and muscle aches, particularly during flu season.
Urinary tract infection (UTI): Can cause fever, especially in women, along with symptoms like dysuria, frequency, and urgency.
Gastroenteritis (stomach flu): Fever can be a symptom, along with diarrhea, vomiting, and abdominal cramps.

Meningitis: Although less common, it is critical to consider due to its high morbidity and mortality if not promptly treated. Symptoms can include fever, headache, stiff neck, and confusion.
Sepsis: A life-threatening condition that arises when the body's response to infection causes injury to its own tissues and organs. Fever is a key symptom, along with tachycardia, tachypnea, and potential hypotension.
Appendicitis: Abdominal pain, fever, and vomiting are common presentations. It requires urgent surgical intervention to prevent perforation.
Pneumonia: Can present with fever, cough, and difficulty breathing. It's a serious condition, especially in the elderly, young children, and those with compromised immune systems.

Tuberculosis (TB): Although rare in many parts of the world, TB can cause a chronic fever, along with weight loss, night sweats, and cough.
Endocarditis: An infection of the heart valves, which can cause fever, new heart murmur, and embolic phenomena.
Brucellosis: A zoonotic infection that can cause undulant fever, malaise, and weight loss, typically in individuals who work with animals or consume unpasteurized dairy products.
Lymphoma: Certain types of lymphoma can present with prolonged fever, night sweats, and weight loss, among other symptoms.