The most common cause for a fever in adults is an infection. Cells that fight infection produce chemicals that stimulate the brain to raise the body temperature.

Common causes of fever in adults include:

• Bronchitis
• Colds
• Gastroenteritis
• Influenza
• Mononucleosis
• Otitis media
• Otitis media
• Pelvic inflammatory disease
• Pharyngitis
• Pneumonia
• Prostatitis
• Sinusitis
• Tonsillitis
• Urinary tract infection
• Viral syndrome

Less common causes of fever in adults include:

• Acute appendicitis
• AIDS
• Alcohol withdrawal
• Autoimmune disease:
• Crohn's disease
• Lupus
• Rheumatoid arthritis
• Ulcerative colitis
• Bacterial endocarditis
• Cancer
• Chicken pox
• Drug side effect causing fever
• Drug withdrawal
• Encephalitis
• Familial Mediterranean fever
• Gallbladder disease
• Hepatitis A
• Hepatitis B
• Hepatitis C
• Histoplasmosis
• HIV infection
• Legionnaire's disease
• Leukemia
• Lung abscess
• Lyme disease
• Lymphoma
• Malaria
• Measles
• Meningitis
• Mumps
• Perirectal abscess
• Pyelonephritis
• Rubella
• Salmonella infection
• Scarlet fever
• Sepsis
• Syphilis
• Tuberculosis
• Tularemia
• Toxic shock syndrome
• Transfusion reaction
• Typhoid
• Typhus
• Vaccine reaction