Causes of pyrexia may be infectious or non-infectious. Some of the common reasons of pyrexia are listed here: Infectious Causes: Lower respiratory tract infections like bronchitis (inflammation of the air tubules that carry blood in and out of the lungs) A bacterial lung infection called tuberculosis Complicated urinary tract infections Bone infections like Osteomyelitis A bacterial infection of the cardiac tissue called endocarditis Viral infections like HIV (Human-Immunodeficiency syndrome) and Cytomegalovirus Non-infectious Causes: Neurological conditions like brain fever or haemorrhages Malignant conditions like Leukaemia and renal cell carcinoma Reactions to drugs Bowel or bladder related problems Reactions to blood transfusion